avfilter/signature_lookup: Fix 2 differences to the refernce SW

Fixes: CID1403227 Division or modulo by float zero

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 25cb66369e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/signature_lookup.c

@@ -501,10 +501,10 @@ static MatchingInfo evaluate_parameters(AVFilterContext *ctx, SignatureContext *
             continue; /* matching sequence is too short */
         if ((double) goodfcount / (double) fcount < sc->thit)
             continue;
-        if ((double) goodfcount*0.5 < FFMAX(gooda, goodb))
+        if ((double) goodfcount*0.5 <= FFMAX(gooda, goodb))
             continue;
 
-        meandist = (double) goodfcount / (double) distsum;
+        meandist = (double) distsum / (double) goodfcount;
 
         if (meandist < minmeandist ||
                 status == (STATUS_END_REACHED | STATUS_BEGIN_REACHED) ||