Add missing length checks to make sure we don't dereference a pointer

Add missing length checks to make sure we don't dereference a pointer
past the mmap(2)'d buffer.  Otherwise, locate will read a single
byte past the end of the buffer.  This is often harmless, but if
the length of the buffer is an even multiple of the page size,
locate will crash.  OK tb@ espie@ deraadt@