lavf/img2dec: fix weird start_number count logic

This seems consistent with the assumed meaning of the option.

libavformat/img2dec.c

@@ -253,7 +253,7 @@ static int read_header(AVFormatContext *s1)
 #endif
         } else {
             if (find_image_range(&first_index, &last_index, s->path,
-                                 s->start_number - 1) < 0)
+                                 s->start_number) < 0)
                 return AVERROR(ENOENT);
         }
         s->img_first = first_index;
@@ -377,7 +377,7 @@ static const AVOption options[] = {
     { "video_size",   "", OFFSET(video_size),   AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, DEC },
     { "framerate",    "", OFFSET(framerate),    AV_OPT_TYPE_STRING, {.str = "25"}, 0, 0, DEC },
     { "loop",         "", OFFSET(loop),         AV_OPT_TYPE_INT,    {.dbl = 0},    0, 1, DEC },
-    { "start_number", "first number in the sequence", OFFSET(start_number), AV_OPT_TYPE_INT, {.dbl = 1}, 1, INT_MAX, DEC },
+    { "start_number", "first number in the sequence", OFFSET(start_number), AV_OPT_TYPE_INT, {.dbl = 0}, 0, INT_MAX, DEC },
     { NULL },
 };