[qcommon] Use unsigned types where wrapping arithmetic is intended

The use of signed types in these expressions lead to overflow, hence undefined behaviour. The "sum" aggregator in Com_TouchMemory isn't even used (and presumbably just exists to inhibit optimizations from removing the memory access).

code/qcommon/common.c

@@ -1376,7 +1376,7 @@ Touch all known used data to make sure it is paged in
 void Com_TouchMemory( void ) {
 	int		start, end;
 	int		i, j;
-	int		sum;
+	unsigned	sum;
 	memblock_t	*block;
 
 	Z_CheckHeap();