avformat/hlsenc: Don't segfault on uncommon names

avformat/hlsenc: Don't segfault on uncommon names

The parsing process of the AVOpt-enabled string controlling the mapping
of input streams to variant streams is roughly as follows: Space and tab
separate variant stream group maps while the entries in each variant
stream group map are separated by ','.

The parsing process of each variant stream group proceeded as follows:
At first the number of occurences of "a:", "v:" and "s:" in each variant
stream group is calculated so that one can can allocate an array of
streams with this number of entries. Then the string is split along ','
and each substring is parsed. If such a substring starts with "a:", "s:"
or "v:" it is treated as stream specifier and (if there is a correct
number after ':') a stream of the variant stream is mapped to one of the
actual input streams.

Nothing actually guarantees that the number of streams allocated initially
equals the number of streams that are mapped to an actual input stream.
These numbers can differ if e.g. the name, the sgroup, agroup or ccgroup
of the variant stream contain "a:", "s:" or "v:".

The problem hereby is that the rest of the code presumes these numbers
to be equal and segfaults if it isn't (because the corresponding input
stream is NULL).

This commit fixes this by modifying the initial counting process to only
count occurences of "a:", "s:" or "v:" that are at the beginning or that
immediately follow a ','.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>