vmd: refactor the inner decode loop

Simplify a little, assume empty frames are acceptable and
do not pointlessly reinit the bytestream2 contexts using
possibly wrong size values.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/vmdav.c

@@ -268,9 +268,11 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
         }
         s->size -= PALETTE_COUNT * 3 + 2;
     }
-    if (s->size > 0) {
+
+    if (!s->size)
+        return 0;
+
     /* originally UnpackFrame in VAG's code */
-        bytestream2_init(&gb, gb.buffer, s->buf + s->size - gb.buffer);
     if (bytestream2_get_bytes_left(&gb) < 1)
         return AVERROR_INVALIDDATA;
     meth = bytestream2_get_byteu(&gb);
@@ -291,7 +293,8 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
                 len = bytestream2_get_byte(&gb);
                 if (len & 0x80) {
                     len = (len & 0x7F) + 1;
-                        if (ofs + len > frame_width || bytestream2_get_bytes_left(&gb) < len)
+                    if (ofs + len > frame_width ||
+                        bytestream2_get_bytes_left(&gb) < len)
                         return AVERROR_INVALIDDATA;
                     bytestream2_get_buffer(&gb, &dp[ofs], len);
                     ofs += len;
@@ -304,7 +307,8 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
                 }
             } while (ofs < frame_width);
             if (ofs > frame_width) {
-                    av_log(s->avctx, AV_LOG_ERROR, "VMD video: offset > width (%d > %d)\n",
+                av_log(s->avctx, AV_LOG_ERROR,
+                       "VMD video: offset > width (%d > %d)\n",
                        ofs, frame_width);
                 return AVERROR_INVALIDDATA;
             }
@@ -344,7 +348,8 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
                 }
             } while (ofs < frame_width);
             if (ofs > frame_width) {
-                    av_log(s->avctx, AV_LOG_ERROR, "VMD video: offset > width (%d > %d)\n",
+                av_log(s->avctx, AV_LOG_ERROR,
+                       "VMD video: offset > width (%d > %d)\n",
                        ofs, frame_width);
                 return AVERROR_INVALIDDATA;
             }
@@ -353,7 +358,6 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
         }
         break;
     }
-    }
     return 0;
 }