rtpdec_jpeg: fix low contrast image on low quality setting

The problem is that the argument 'q' is of the type uint8_t.
According to the JPEG standard, if 1 <= q <= 50, the scale factor
'S' should be 5000 / Q. Because the create_default_qtables() reuses
the variable 'q' to store the result of this calculation, for small
values of q < 19, q wil subsequently overflow and give wrong results
in the calculated quantization tables.

Instead, use a new variable 'S' (same name as in RFC2435) with the
proper range to store the result of the division.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec_jpeg.c

@@ -186,16 +186,17 @@ static void create_default_qtables(uint8_t *qtables, uint8_t q)
 {
     int factor = q;
     int i;
+    uint16_t S;
 
     factor = av_clip(q, 1, 99);
 
     if (q < 50)
-        q = 5000 / factor;
+        S = 5000 / factor;
     else
-        q = 200 - factor * 2;
+        S = 200 - factor * 2;
 
     for (i = 0; i < 128; i++) {
-        int val = (default_quantizers[i] * q + 50) / 100;
+        int val = (default_quantizers[i] * S + 50) / 100;
 
         /* Limit the quantizers to 1 <= q <= 255. */
         val = av_clip(val, 1, 255);