avformat/dashenc: Fix leak of AVFormatContext on error

The Dash muxer uses submuxers and when one such submuxer has been allocated,
it is initially only stored in a temporary variable. Therefore it leaks
if an error happens between the allocation and storing it permanently.
This commit changes this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: "Jeyapal, Karthick" <kjeyapal@akamai.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/dashenc.c

@@ -1225,10 +1225,6 @@ static int dash_init(AVFormatContext *s)
         dict_copy_entry(&as->metadata, s->streams[i]->metadata, "language");
         dict_copy_entry(&as->metadata, s->streams[i]->metadata, "role");
 
-        ctx = avformat_alloc_context();
-        if (!ctx)
-            return AVERROR(ENOMEM);
-
         if (c->init_seg_name) {
             os->init_seg_name = av_strireplace(c->init_seg_name, "$ext$", os->extension_name);
             if (!os->init_seg_name)
@@ -1261,10 +1257,13 @@ static int dash_init(AVFormatContext *s)
             }
         }
 
+        os->ctx = ctx = avformat_alloc_context();
+        if (!ctx)
+            return AVERROR(ENOMEM);
+
         ctx->oformat = av_guess_format(os->format_name, NULL, NULL);
         if (!ctx->oformat)
             return AVERROR_MUXER_NOT_FOUND;
-        os->ctx = ctx;
         ctx->interrupt_callback    = s->interrupt_callback;
         ctx->opaque                = s->opaque;
         ctx->io_close              = s->io_close;