avfilter/vf_fps: Work around msvc (c99wrap) build failure

c99wrap choked on initialization of .dbl start_time option with
AV_NOPTS_VALUE: Unable to parse int64_t as expression primary
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_fps.c

@@ -61,7 +61,7 @@ typedef struct FPSContext {
 #define F AV_OPT_FLAG_FILTERING_PARAM
 static const AVOption fps_options[] = {
     { "fps", "A string describing desired output framerate", OFFSET(framerate), AV_OPT_TYPE_VIDEO_RATE, { .str = "25" }, .flags = V|F },
-    { "start_time", "Assume the first PTS should be this value.", OFFSET(start_time), AV_OPT_TYPE_DOUBLE, { .dbl = AV_NOPTS_VALUE}, INT64_MIN, INT64_MAX, V },
+    { "start_time", "Assume the first PTS should be this value.", OFFSET(start_time), AV_OPT_TYPE_DOUBLE, { .dbl = -9223372036854775808.0}, INT64_MIN, INT64_MAX, V },
     { "round", "set rounding method for timestamps", OFFSET(rounding), AV_OPT_TYPE_INT, { .i64 = AV_ROUND_NEAR_INF }, 0, 5, V|F, "round" },
     { "zero", "round towards 0",      OFFSET(rounding), AV_OPT_TYPE_CONST, { .i64 = AV_ROUND_ZERO     }, 0, 5, V|F, "round" },
     { "inf",  "round away from 0",    OFFSET(rounding), AV_OPT_TYPE_CONST, { .i64 = AV_ROUND_INF      }, 0, 5, V|F, "round" },