Fix GC_CHECK_STRING_BYTES false alarm with pdumper

* src/alloc.c (string_bytes): Don’t abort on strings
taken from the dumped file.

src/alloc.c

@@ -1851,7 +1851,7 @@ string_bytes (struct Lisp_String *s)
   ptrdiff_t nbytes =
     (s->u.s.size_byte < 0 ? s->u.s.size & ~ARRAY_MARK_FLAG : s->u.s.size_byte);
 
-  if (!PURE_P (s) && s->u.s.data
+  if (!PURE_P (s) && !pdumper_object_p (s) && s->u.s.data
       && nbytes != SDATA_NBYTES (SDATA_OF_STRING (s)))
     emacs_abort ();
   return nbytes;