hlsenc: Allocate enough space for the pattern string

If s->filename doesn't contain any period/filename extension to strip
away, the buffer will be too small to fit both strings. This isn't
any buffer overflow since the concatenation uses av_strlcat with
the right buffer size.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/hlsenc.c

@@ -182,7 +182,7 @@ static int hls_write_header(AVFormatContext *s)
     int ret, i;
     char *p;
     const char *pattern = "%d.ts";
-    int basename_size = strlen(s->filename) + strlen(pattern);
+    int basename_size = strlen(s->filename) + strlen(pattern) + 1;
 
     hls->number      = 0;