[webvtt] crash on jump

Steps to reproduce:

1. Open mp4 file.
2. Open subtitles file.
3. Jump on timeline

Subtitles file content:

WEBVTT

.1
00:01:10,113 --> 00:01:10,500
.

Thread 20 "vlc" received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x7fff92d3e700 (LWP 22527)] 0x00007fff3bdea8b6 in Control (p_demux=0x7fff8d51d340, i_query=772, args=0x7fff92d3db70) at demux/webvtt.c:444 444 p_sys->index.p_array[p_sys->index.i_current].time;

all malformed cues

13408f65

added Severity::normal Status::fixed labels

closed