VP8LSubtractGreenFromBlueAndRed_C: clear int sanitizer warnings

previously the types were changed to int to prevent unsigned overflow
warnings:
6ab496e fix some 'unsigned integer overflow' warnings in ubsan

clears warnings of the form:
implicit conversion from type 'uint32_t' (aka 'unsigned int') of value
3724541952 (32-bit, unsigned) to type 'int' changed the value to
-570425344 (32-bit, signed)

implicit conversion from type 'int' of value -3361661 (32-bit, signed)
to type 'unsigned int' changed the value to 4291605635 (32-bit,
unsigned)

Bug: b/229626362
Change-Id: If1eb39c5dd7218d686c3c47fb7df72431b873be4

src/dsp/lossless_enc.c

@@ -522,11 +522,11 @@ static void GetCombinedEntropyUnrefined_C(const uint32_t X[],
 void VP8LSubtractGreenFromBlueAndRed_C(uint32_t* argb_data, int num_pixels) {
   int i;
   for (i = 0; i < num_pixels; ++i) {
-    const int argb = argb_data[i];
+    const int argb = (int)argb_data[i];
     const int green = (argb >> 8) & 0xff;
     const uint32_t new_r = (((argb >> 16) & 0xff) - green) & 0xff;
     const uint32_t new_b = (((argb >>  0) & 0xff) - green) & 0xff;
-    argb_data[i] = (argb & 0xff00ff00u) | (new_r << 16) | new_b;
+    argb_data[i] = ((uint32_t)argb & 0xff00ff00u) | (new_r << 16) | new_b;
   }
 }