add positive_mod() utility function.

source/blender/blenlib/BLI_math_base.h

@@ -222,6 +222,7 @@ MINLINE int power_of_2_max_i(int n);
 MINLINE int power_of_2_min_i(int n);
 
 MINLINE int divide_round_i(int a, int b);
+MINLINE int positive_mod(int i, int n);
 
 MINLINE float shell_angle_to_dist(const float angle);
 

source/blender/blenlib/intern/math_base_inline.c

@@ -153,6 +153,11 @@ MINLINE int divide_round_i(int a, int b)
 	return (2 * a + b) / (2 * b);
 }
 
+MINLINE int positive_mod(int i, int n)
+{
+	return ((i = i % n) < 0) ? i + n : i;
+}
+
 MINLINE unsigned int highest_order_bit_i(unsigned int n)
 {
 	n |= (n >>  1);

source/blender/bmesh/operators/bmo_bridge.c

@@ -273,8 +273,7 @@ static void bridge_loop_pair(BMesh *bm,
 		if (twist_offset != 0) {
 			const int len_b = BM_edgeloop_length_get(el_store_b);
 			ListBase *lb_b = BM_edgeloop_verts_get(el_store_b);
-			const int offset = twist_offset % len_b;
-			LinkData *el_b = BLI_rfindlink(lb_b, (offset < 0) ? (offset + len_b) : offset);
+			LinkData *el_b = BLI_rfindlink(lb_b, positive_mod(twist_offset, len_b));
 			BLI_rotatelist(lb_b, el_b);
 		}
 	}