avformat/sbgdec: Check for negative duration

Fixes: signed integer overflow: 9223372036854775807 - -8000000 cannot be represented in type 'long'
Fixes: 62276/clusterfuzz-testcase-minimized-ffmpeg_dem_SBG_fuzzer-5133181743136768

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/sbgdec.c

@@ -387,7 +387,7 @@ static int parse_options(struct sbg_parser *p)
                 case 'L':
                     FORWARD_ERROR(parse_optarg(p, opt, &oarg));
                     r = str_to_time(oarg.s, &p->scs.opt_duration);
-                    if (oarg.e != oarg.s + r) {
+                    if (oarg.e != oarg.s + r || p->scs.opt_duration < 0) {
                         snprintf(p->err_msg, sizeof(p->err_msg),
                                  "syntax error for option -L");
                         return AVERROR_INVALIDDATA;