getenv: Move call to strlen to the branch it's used in.

The len variable is only used in the else branch.
We don't need the call to strlen if the name is 0 or 1 characters long.

2019-10-02  Lode Willems  <Lode.Willems@UGent.be>

	* tdlib/getenv.c: Move the call to strlen into the branch it's used.

stdlib/getenv.c

@@ -32,7 +32,6 @@
 char *
 getenv (const char *name)
 {
-  size_t len = strlen (name);
   char **ep;
   uint16_t name_start;
 
@@ -63,6 +62,7 @@ getenv (const char *name)
     }
   else
     {
+      size_t len = strlen (name);
 #if _STRING_ARCH_unaligned
       name_start = *(const uint16_t *) name;
 #else