avformat/hlsenc: Improve checks for invalid stream mappings

avformat/hlsenc: Improve checks for invalid stream mappings

The mapping of streams to the various variant streams to be created by
the HLS muxer 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 each entry is checked and the
check for stream numbers was deficient: It did check that there is a
number beginning after the ":", but it did not check that the number
extends until the next "," (or until the end).

This means that an invalid variant stream group like v:0_v:1 will not be
rejected; the problem is that the variant stream in this example is
supposed to have two streams associated with it (because it contains two
"v:"), yet only one stream is actually associated with it (because there
is no ',' to start a second stream specifier). This discrepancy led to
segfaults (null pointer dereferencing) in the rest of the code (when the
nonexistent second stream associated to the variant stream was inspected).

Furthermore, this commit also removes an instance of using atoi() whose
behaviour on a range error is undefined.

Fixes ticket #8652.

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