hlsenc: Allocate enough space for the pattern string

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>