Quell divide-by-zero warning

Coverity has been noting that `avg` might be zero when dividing. This is impossible.

Use assert() to quell the Coverity warning.

Closes CID 13801515

src/sdl/utils.cpp

@@ -1628,6 +1628,7 @@ surface blur_alpha_surface(const surface &surf, int depth)
 
 		p = lock.pixels() + y*res->w;
 		for(x = 0; x < res->w; ++x, ++p) {
+			assert(avg);
 			*p = (std::min(alpha/avg,ff) << 24) | (std::min(red/avg,ff) << 16) | (std::min(green/avg,ff) << 8) | std::min(blue/avg,ff);
 			if(x >= depth) {
 				alpha -= ((*front) >> 24)&0xFF;