lavc/gifdec: move idx_line allocation out of gif_read_header1()

Now allocation is done only if av_image_check_size() passes.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/gifdec.c

@@ -376,10 +376,6 @@ static int gif_read_header1(GifState *s)
         return AVERROR_INVALIDDATA;
     }
 
-    av_fast_malloc(&s->idx_line, &s->idx_line_size, s->screen_width);
-    if (!s->idx_line)
-        return AVERROR(ENOMEM);
-
     v = bytestream2_get_byteu(&s->gb);
     s->color_resolution = ((v & 0x70) >> 4) + 1;
     s->has_global_palette = (v & 0x80);
@@ -486,6 +482,10 @@ static int gif_decode_frame(AVCodecContext *avctx, void *data, int *got_frame, A
             return ret;
         }
 
+        av_fast_malloc(&s->idx_line, &s->idx_line_size, s->screen_width);
+        if (!s->idx_line)
+            return AVERROR(ENOMEM);
+
         s->picture.pict_type = AV_PICTURE_TYPE_I;
         s->picture.key_frame = 1;
         s->keyframe_ok = 1;