avcodec/snowenc: fix SIGFPE in get_dc() when a block lies outside the plane

get_dc() divides the accumulated, OBMC-weighted DC by aa, the sum of the
squared OBMC weights taken over the in-plane pixels.  When an OBMC block
falls entirely outside the plane - e.g. a tiny chroma plane after mcdeint
splits a frame into fields - no pixel contributes, aa stays 0 and the
ROUNDED_DIV() divides by zero (SIGFPE).  ab is 0 in exactly the same case,
so the result degenerates to 0; return it directly.

Reproducible with the GPL mcdeint filter in slow/extra_slow mode, e.g.

    ffmpeg -f lavfi -i testsrc=s=128x2 -vf mcdeint=mode=slow -f null -

Add a self-contained lavfi-based FATE regression test for the slow mode,
which previously crashed and is therefore not covered by the existing
sample-based fast/medium tests.

Fixes trac ticket #7779.

Signed-off-by: Bogdan Lisman <bogdan@pydevsolutions.com>
(cherry picked from commit a62d996927)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/snowenc.c

@@ -564,6 +564,9 @@ static int get_dc(SnowContext *s, int mb_x, int mb_y, int plane_index){
     }
     *b= backup;
 
+    if (!aa)
+        return 0;
+
     return av_clip_uint8( ROUNDED_DIV(ab<<LOG2_OBMC_MAX, aa) ); //FIXME we should not need clipping
 }