rtjpeg: check get_block return value for error.

This avoids crashes due to reading out-of-bounds.
Signed-off-by: NAnton Khirnov <anton@khirnov.net>

libavcodec/rtjpeg.c

@@ -114,24 +114,25 @@ int rtjpeg_decode_frame_yuv420(RTJpegContext *c, AVFrame *f,
     init_get_bits(&gb, buf, buf_size * 8);
     for (y = 0; y < h; y++) {
         for (x = 0; x < w; x++) {
+#define BLOCK(quant, dst, stride) do { \
+    int res = get_block(&gb, block, c->scan, quant); \
+    if (res < 0) \
+        return res; \
+    if (res > 0) \
+        c->dsp->idct_put(dst, stride, block); \
+} while (0)
             DCTELEM *block = c->block;
-            if (get_block(&gb, block, c->scan, c->lquant) > 0)
-                c->dsp->idct_put(y1, f->linesize[0], block);
+            BLOCK(c->lquant, y1, f->linesize[0]);
             y1 += 8;
-            if (get_block(&gb, block, c->scan, c->lquant) > 0)
-                c->dsp->idct_put(y1, f->linesize[0], block);
+            BLOCK(c->lquant, y1, f->linesize[0]);
             y1 += 8;
-            if (get_block(&gb, block, c->scan, c->lquant) > 0)
-                c->dsp->idct_put(y2, f->linesize[0], block);
+            BLOCK(c->lquant, y2, f->linesize[0]);
             y2 += 8;
-            if (get_block(&gb, block, c->scan, c->lquant) > 0)
-                c->dsp->idct_put(y2, f->linesize[0], block);
+            BLOCK(c->lquant, y2, f->linesize[0]);
             y2 += 8;
-            if (get_block(&gb, block, c->scan, c->cquant) > 0)
-                c->dsp->idct_put(u, f->linesize[1], block);
+            BLOCK(c->cquant, u,  f->linesize[1]);
             u += 8;
-            if (get_block(&gb, block, c->scan, c->cquant) > 0)
-                c->dsp->idct_put(v, f->linesize[2], block);
+            BLOCK(c->cquant, v,  f->linesize[2]);
             v += 8;
         }
         y1 += 2 * 8 * (f->linesize[0] - w);