* gmalloc.c: Fix bug that causes malloc & free to

* gmalloc.c:  Fix bug that causes malloc & free to
fail on systems where pointers have the high bit set (0x800efcf0
for example).  The problem is that the difference between two
pointers is a signed integer, so the computation

	(char *) 0x800efcf0 - (char *) 0

yields a negative value.  The sign of the result of the modulus
operator is machine dependent for negative operands, thus it is
possible for it to end up negative.  From Fred Fish.