Get rid of UB found by UBSan

src/num.c

@@ -1457,7 +1457,8 @@ static void bc_num_parseDecimal(BcNum *restrict n, const char *restrict val) {
 
 	for (i = 0; i < len && (zero = (val[i] == '0' || val[i] == '.')); ++i);
 
-	n->scale = (size_t) (rdx * ((val + len) - (ptr + 1)));
+	n->scale = (size_t) (rdx * (((uintptr_t) (val + len)) -
+	                            (((uintptr_t) ptr) + 1)));
 	n->rdx = BC_NUM_RDX(n->scale);
 
 	i = len - (ptr == val ? 0 : i) - rdx;