varasm: Fix type confusion bug

This patch fixes a type confusion bug in varasm.cc:assemble_variable.
The problem is that the current code calls:

  sect = get_variable_section (decl, false);

and then accesses sect->named.name without checking whether the section
is in fact a named section. In the surrounding else clause, we only know
that SECTION_STYLE (sect) != SECTION_NOSWITCH, so it is possible that
the section is an unnamed section.

In practice, this means that we end up doing a wild string compare
between a function pointer and the string literal ".vtable_map_vars".
This is because sect->named.name aliases sect->unnamed.callback in the
section union.

This can be seen in GDB with a simple testcase such as "int x;".

This patch fixes the issue by checking the SECTION_STYLE of the section
is in fact SECTION_NAMED before trying to do the string comparison.

We drop the existing check of whether sect->named.name is non-NULL
because this should presumably always be the case for a named section.

gcc/ChangeLog:

	* varasm.cc (assemble_variable): Fix type confusion bug when
	checking for ".vtable_map_vars" section.

gcc/varasm.cc

@@ -2400,7 +2400,7 @@ assemble_variable (tree decl, int top_level ATTRIBUTE_UNUSED,
   else
     {
       /* Special-case handling of vtv comdat sections.  */
-      if (sect->named.name
+      if (SECTION_STYLE (sect) == SECTION_NAMED
 	  && (strcmp (sect->named.name, ".vtable_map_vars") == 0))
 	handle_vtv_comdat_section (sect, decl);
       else