stb_image: Reject fractional JPEG component subsampling ratios

The component resamplers are not written to support this and I've
never seen it happen in a real (non-crafted) JPEG file so I'm
fine rejecting this as outright corrupt.

Fixes issue #1178.

stb_image.h

@@ -3267,6 +3267,13 @@ static int stbi__process_frame_header(stbi__jpeg *z, int scan)
       if (z->img_comp[i].v > v_max) v_max = z->img_comp[i].v;
    }
 
+   // check that plane subsampling factors are integer ratios; our resamplers can't deal with fractional ratios
+   // and I've never seen a non-corrupted JPEG file actually use them
+   for (i=0; i < s->img_n; ++i) {
+      if (h_max % z->img_comp[i].h != 0) return stbi__err("bad H","Corrupt JPEG");
+      if (v_max % z->img_comp[i].v != 0) return stbi__err("bad V","Corrupt JPEG");
+   }
+
    // compute interleaved mcu info
    z->img_h_max = h_max;
    z->img_v_max = v_max;