Merge pull request #16138 from pmur:reg_16137

* imgproc: Prevent 1B overrun of 8C3 SIMD optimization

The fourth value read via v_load_q is essentially ignored,
but can cause trouble if it happens to cross page boundaries.

The final few iterations may attempt to read the most extreme
elements of S, which will read 1B beyond the array in most
aligment cases. Dynamically compute the stop. This could be
hoised from the loop, but will require a more extensive change.

Likewise, cleanup the iteration increment statements to make
it more obvious they do channel count (3) elements per pass.

This should resolve #16137

* imgproc(resize): extra check

modules/imgproc/src/resize.cpp

@@ -1547,7 +1547,7 @@ struct HResizeLinearVec_X4
 struct HResizeLinearVecU8_X4
 {
     int operator()(const uchar** src, uchar** _dst, int count, const int* xofs,
-        const uchar* _alpha, int, int, int cn, int, int xmax) const
+        const uchar* _alpha, int smax, int, int cn, int, int xmax) const
     {
         const short *alpha = (const short*)_alpha;
         int **dst = (int**)_dst;
@@ -1633,8 +1633,18 @@ struct HResizeLinearVecU8_X4
         }
         else if(cn == 3)
         {
-            const int step = 4;
-            const int len0 = xmax - step;
+            int len0 = xmax - cn;
+
+            /* This may need to trim 1 or more extra units depending on the amount of
+               scaling. Test until we find the first value which we know cannot overrun. */
+            while (len0 >= cn &&
+                xofs[len0 - cn] + cn >= smax - cn  // check access: v_load_expand_q(S+xofs[dx]+cn)
+            )
+            {
+                len0 -= cn;
+            }
+            CV_DbgAssert(len0 <= 0 || len0 >= cn);
+
             for( ; k <= (count - 2); k+=2 )
             {
                 const uchar *S0 = src[k];
@@ -1642,7 +1652,7 @@ struct HResizeLinearVecU8_X4
                 const uchar *S1 = src[k+1];
                 int *D1 = dst[k+1];
 
-                for( dx = 0; dx < len0; dx += 3*step/4 )
+                for( dx = 0; dx < len0; dx += cn )
                 {
                     v_int16x8 a = v_load(alpha+dx*2);
                     v_store(&D0[dx], v_dotprod(v_reinterpret_as_s16(v_load_expand_q(S0+xofs[dx]) | (v_load_expand_q(S0+xofs[dx]+cn)<<16)), a));
@@ -1653,7 +1663,7 @@ struct HResizeLinearVecU8_X4
             {
                 const uchar *S = src[k];
                 int *D = dst[k];
-                for( dx = 0; dx < len0; dx += 3*step/4 )
+                for( dx = 0; dx < len0; dx += cn )
                 {
                     v_int16x8 a = v_load(alpha+dx*2);
                     v_store(&D[dx], v_dotprod(v_reinterpret_as_s16(v_load_expand_q(S+xofs[dx]) | (v_load_expand_q(S+xofs[dx]+cn)<<16)), a));