The mcheck wrapper for realloc() unconditionally frees memory and
returns NULL when size is 0. This differs from dlmalloc's default
behaviour which returns a minimum-sized allocation unless
REALLOC_ZERO_BYTES_FREES is defined.
Make the mcheck wrapper respect the same REALLOC_ZERO_BYTES_FREES
setting for consistent behavior with or without mcheck enabled.
Signed-off-by: Simon Glass <simon.glass@canonical.com>