lagarith: reallocate rgb_planes when needed

Fixes invalid writes on pixel format changes.

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

libavcodec/lagarith.c

@@ -52,6 +52,7 @@ typedef struct LagarithContext {
     int zeros;                  /**< number of consecutive zero bytes encountered */
     int zeros_rem;              /**< number of zero bytes remaining to output */
     uint8_t *rgb_planes;
+    int      rgb_planes_allocated;
     int rgb_stride;
 } LagarithContext;
 
@@ -564,13 +565,12 @@ static int lag_decode_frame(AVCodecContext *avctx,
         offs[1] = offset_gu;
         offs[2] = offset_ry;
 
+        l->rgb_stride = FFALIGN(avctx->width, 16);
+        av_fast_malloc(&l->rgb_planes, &l->rgb_planes_allocated,
+                       l->rgb_stride * avctx->height * planes + 1);
         if (!l->rgb_planes) {
-            l->rgb_stride = FFALIGN(avctx->width, 16);
-            l->rgb_planes = av_malloc(l->rgb_stride * avctx->height * planes + 1);
-            if (!l->rgb_planes) {
-                av_log(avctx, AV_LOG_ERROR, "cannot allocate temporary buffer\n");
-                return AVERROR(ENOMEM);
-            }
+            av_log(avctx, AV_LOG_ERROR, "cannot allocate temporary buffer\n");
+            return AVERROR(ENOMEM);
         }
         for (i = 0; i < planes; i++)
             srcs[i] = l->rgb_planes + (i + 1) * l->rgb_stride * avctx->height - l->rgb_stride;