loader/multiboot_elfxx: Check program memory isn't larger than allocated memory size

loader/multiboot_elfxx: Check program memory isn't larger than allocated memory size

In grub-core/loader/multiboot_elfxx.c, the code is filling an area of memory
with grub_memset() but doesn't check if there is space in the allocated memory
before doing so. To make sure we aren't zeroing memory past the allocated memory
region, we need to check that the offset into the allocated memory region plus
the memory size of the program is smaller than the allocated memory size.

Fixes: CID 314029
Fixes: CID 314038
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>