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>