avcodec/iff: warn about truncated input to decode_byterun() and clear remaining output

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fe205e395ed_8988_test.iff
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/iff.c

@@ -499,6 +499,10 @@ static int decode_byterun(uint8_t *dst, int dst_size,
         }
         x += length;
     }
+    if (x < dst_size) {
+        av_log(NULL, AV_LOG_WARNING, "decode_byterun ended before plane size\n");
+        memset(dst+x, 0, dst_size - x);
+    }
     return buf - buf_start;
 }