avfilter/vf_signature: Fix leak of string upon error

If an error happens between allocating a string intended to be used as
an inpad's name and attaching it to its input pad, the string leaks.
Fix this by inserting the inpad directly after allocating its string.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/vf_signature.c

@@ -663,6 +663,10 @@ static av_cold int init(AVFilterContext *ctx)
 
         if (!pad.name)
             return AVERROR(ENOMEM);
+        if ((ret = ff_insert_inpad(ctx, i, &pad)) < 0) {
+            av_freep(&pad.name);
+            return ret;
+        }
 
         sc = &(sic->streamcontexts[i]);
 
@@ -679,11 +683,6 @@ static av_cold int init(AVFilterContext *ctx)
         sc->coarseend = sc->coarsesiglist;
         sc->coarsecount = 0;
         sc->midcoarse = 0;
-
-        if ((ret = ff_insert_inpad(ctx, i, &pad)) < 0) {
-            av_freep(&pad.name);
-            return ret;
-        }
     }
 
     /* check filename */