malloc_simple: Return NULL on malloc failure rather then calling panic()

All callers of malloc should already do error checking, and may even be able
to continue without the alloc succeeding.

Moreover, common/malloc_simple.c is the only user of .rodata.str1.1 in
common/built-in.o when building the SPL, triggering this gcc bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54303

Causing .rodata to grow with e.g. 0xc21 bytes, nullifying all benefits of
using malloc_simple in the first place.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>

common/malloc_simple.c

@@ -19,7 +19,7 @@ void *malloc_simple(size_t bytes)
 
 	new_ptr = gd->malloc_ptr + bytes;
 	if (new_ptr > gd->malloc_limit)
-		panic("Out of pre-reloc memory");
+		return NULL;
 	ptr = map_sysmem(gd->malloc_base + gd->malloc_ptr, bytes);
 	gd->malloc_ptr = ALIGN(new_ptr, sizeof(new_ptr));
 	return ptr;