mmap/mmap: Add missing grub_malloc() failure check

mmap/mmap: Add missing grub_malloc() failure check

This patch adds a NULL check after grub_malloc() call. Missing a failure
check after calling grub_malloc() can lead to undefined behavior. If the
allocation fails and returns NULL subsequent dereferencing or writing to
the pointer will likely result in a runtime error such as a segmentation
fault.

Signed-off-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>