fix signed/unsignted compare warning

stb_image.h

@@ -1646,7 +1646,7 @@ stbi_inline static int stbi__extend_receive(stbi__jpeg *j, int n)
 
    sgn = (stbi__int32)j->code_buffer >> 31; // sign bit is always in MSB
    k = stbi_lrot(j->code_buffer, n);
-   STBI_ASSERT(n >= 0 && n < sizeof(stbi__bmask)/sizeof(*stbi__bmask));
+   STBI_ASSERT(n >= 0 && n < (int) (sizeof(stbi__bmask)/sizeof(*stbi__bmask)));
    j->code_buffer = k & ~stbi__bmask[n];
    k &= stbi__bmask[n];
    j->code_bits -= n;