relocator: Protect grub_relocator_alloc_chunk_addr() input args against integer under...

relocator: Protect grub_relocator_alloc_chunk_addr() input args against integer underflow/overflow

Use arithmetic macros from safemath.h to accomplish it. In this commit,
I didn't want to be too paranoid to check every possible math equation
for overflow/underflow. Only obvious places (with non zero chance of
overflow/underflow) were refactored.

Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>