buffersrc: default SAR to 0 (unknown) rather than 1

It makes more sense to not claim anything about the SAR if we don't know
anything. No changes in the FATE tests, since this is what avconv ends
up doing anyway.

libavfilter/buffersrc.c

@@ -190,7 +190,7 @@ static const AVOption video_options[] = {
     { "sar_num",       "deprecated, do not use", OFFSET(pixel_aspect.num), AV_OPT_TYPE_INT,      { .i64 = 0 }, 0, INT_MAX, V },
     { "sar_den",       "deprecated, do not use", OFFSET(pixel_aspect.den), AV_OPT_TYPE_INT,      { .i64 = 0 }, 0, INT_MAX, V },
 #endif
-    { "sar",           "sample aspect ratio",    OFFSET(pixel_aspect),     AV_OPT_TYPE_RATIONAL, { .dbl = 1 }, 0, DBL_MAX, V },
+    { "sar",           "sample aspect ratio",    OFFSET(pixel_aspect),     AV_OPT_TYPE_RATIONAL, { .dbl = 0 }, 0, DBL_MAX, V },
     { "time_base",     NULL,                     OFFSET(time_base),        AV_OPT_TYPE_RATIONAL, { .dbl = 0 }, 0, DBL_MAX, V },
     { "frame_rate",    NULL,                     OFFSET(frame_rate),       AV_OPT_TYPE_RATIONAL, { .dbl = 0 }, 0, DBL_MAX, V },
     { NULL },