Prevent signed integer overflow in LshTable

This change keeps everything as size_t and the expression is valid
because maxk_block > 0 in hte loop.

modules/flann/include/opencv2/flann/lsh_table.h

@@ -424,7 +424,7 @@ inline size_t LshTable<unsigned char>::getKey(const unsigned char* feature) cons
         size_t mask_block = mask_[i / sizeof(size_t)];
         while (mask_block) {
             // Get the lowest set bit in the mask block
-            size_t lowest_bit = mask_block & (-(ptrdiff_t)mask_block);
+            size_t lowest_bit = mask_block & ~(mask_block - 1);
             // Add it to the current subsignature if necessary
             subsignature += (feature_block & lowest_bit) ? bit_index : 0;
             // Reset the bit in the mask block