lavfi: add braces around the block of an if() expression in avfilter_default_get_video_buffer

Clarify code layout.

libavfilter/defaults.c

@@ -42,7 +42,8 @@ AVFilterBufferRef *avfilter_default_get_video_buffer(AVFilterLink *link, int per
     AVFilterBufferRef *picref = NULL;
     AVFilterPool *pool = link->pool;
 
-    if (pool) for (i = 0; i < POOL_SIZE; i++) {
+    if (pool) {
+        for (i = 0; i < POOL_SIZE; i++) {
         picref = pool->pic[i];
         if (picref && picref->buf->format == link->format && picref->buf->w == w && picref->buf->h == h) {
             AVFilterBuffer *pic = picref->buf;
@@ -57,6 +58,7 @@ AVFilterBufferRef *avfilter_default_get_video_buffer(AVFilterLink *link, int per
             memcpy(picref->linesize, pic->linesize, sizeof(picref->linesize));
             return picref;
         }
+        }
     } else
         pool = link->pool = av_mallocz(sizeof(AVFilterPool));