avfilter/REDUCE_FORMATS: fix bug that ended reducing too early

Prior to this it was possible that format reduction was ended
before it fully propagated leading to failure later in picking
formats.
No testcase with unmodified source exists, the case was reproduced
with less aggressive list merging though.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avfiltergraph.c

@@ -616,6 +616,7 @@ do {                                                                   \
                                                                        \
             if (!out_link->in_ ## list->nb) {                          \
                 add_format(&out_link->in_ ##list, fmt);                \
+                ret = 1;                                               \
                 break;                                                 \
             }                                                          \
                                                                        \