Fix a crash trying to save an empty AudioStream

My friends, gather around as I learned something about the C standard
that is horrifying and may keep you, dear reader, up at night.

My journey began trying to fix something entirely unrelated and not
wanting to wait for ubsan builds when changing a testcase. So me, in my
infinite naivete just built the engine with tests=yes, but
optimizations turned on.

This resulted in a segfault on "[Audio][AudioStreamWAV] Save empty file".

Well, then, I thought. Lets built with asan then and find out where this
happens! Would it surprise you, my fellow traveler, that the results
were that no such crash occurred?

Thus, to the debugger I go! Fearless, with great optimism. Where I find
that through many an indirection the crash came because, somehow,
CowData::_unref() was getting called with a _ptr set to 0x1.

This can of course only end in tears. Or segmentation faults as we try
to read an atomic variable at the somewhat inconveniently situated
address at 0xfffffffffffffff0.

So I went and looked at the likely culprit, blaming many an innocent
recent change along the way. I shall spare you the falsly accused. But
if for some reason you slept poorly last night, I can assure you that
the voodoo dolls have been put away and will not be harmed further.

So in AudioStreamWAV::get_data() we go, where we find a perfectly
reasonable function! It checks to see whether or not its data is empty,
and if it is not it will resize a temporary Vector to have data_bytes of
space, after which it will do a perfectly pedestrian memcpy() and all is
well in the world.

Or so it seems! After many an hour of despair and disassembly I, at
last, decided to look at where the data gets set! A breakthrough!
Because of the padding data is never empty! So the code always runs!

Eureka! One would think. But then, foolishly, I looked into the
get_data() function one more. My mortal enemy was staring me in the
face, laughing. Because it did not care about this. Sure, the check was
worthless but still... What are we left with.

At this point I could feel the method mocking me.

"I resize the vector to 0, I then memcpy zero bytes into it." It said,
DARING me to object to this state of affairs.

And yet, if I changed the function to check for "data_bytes" rather
than data.is_empty() no crashes.

Was this a compiler bug? Am I losing my mind? But then... I remembered
the mantra of the wise compiler druids... "It Is Not A Compiler Bug".

But what then! The bug does not happen when memory is being watched!
Valgrind agreed that while accessing the SafeRefCount at
0xfffffffffffffff0 was incredibly rude, it did not inform me of anything
else untoward happening. So I read the memcpy() manpage... nothing... I
read the the memcpy() posix spec... nothing.

Finally, in despair and because I had nothing left to lose... The ISO C
language specification. As I was reading, I could hear
AudioStreamWAV::get_data() cackling, knowing that its time was up, but
proud of the madness it caused in my soul. Knowing I would never be the
same.

The behavior is undefined if either dest or src is an invalid or null pointer.

So... Here I stand before you, a broken person. But one richer in
knowledge.

I write you this from the depths of madness in the hopes that you, dear
reader, can be spared this ordeal.

May god have mercy on our souls.

We trigger the following sequence of events:

* memcpy(null, null, 0) is UB, thus dest and src cannot be null
* we inline the calls to the ctor and dtor
* now we have a function that does something that "proves"
  dest cannot be null
* we inline cowdata::_unref() which does a null check, on something
  that the compiler just convinced itself cannot be null
* the compiler removes the dead code branch where _ptr == nullptr
* we start to do pointer arithmetic on a nullptr and get send to
  uninitialized memory.

Co-Authored-By: Jason Beckmann <jasonabeckmann@gmail.com>

scene/resources/audio_stream_wav.cpp

@@ -587,7 +587,7 @@ void AudioStreamWAV::set_data(const Vector<uint8_t> &p_data) {
 Vector<uint8_t> AudioStreamWAV::get_data() const {
 	Vector<uint8_t> pv;
 
-	if (!data.is_empty()) {
+	if (data_bytes) {
 		pv.resize(data_bytes);
 		memcpy(pv.ptrw(), data.ptr() + DATA_PAD, data_bytes);
 	}