Potential crash in CC708 append_cc function.

[David Hampton]

This function calls the rightsize_buf function, which attempts to reallocate a buffer if there isn't enough space. It is possible, but extremely unlikely, that the reallocation will fail and the buffer will be replaced with a nullptr. Any attempt to dereference that nullptr will crash the frontend.

[David Hampton <mythtv@…>]

In d0356aba3c/mythtv:

tidy: Fix rare null pointer dereference in cc708decoder.cpp.

The clang-tidy "non-null parameter checker" pointed out to possibility
of the cc708 decoder dereferencing through a null pointer. This code
starts with a default buffer size of 512 and then attempts to allocate
a larger buffer when necessary (in this case, if a single subtitle is
more than 512 characters.) If the reallocation fails, the code would
end up storing the nullptr error return into the buffer pointer, and
then dereference it an an attempt to store the new characters. Catch
this rare condition and drop the characters that won't fit into the
existing buffer.

Fixes #13436.

[David Hampton <mythtv@…>]

In ff4c7158f/mythtv:

tidy: Fix rare null pointer dereference in cc707decoder.cpp.

The clang-tidy "non-null parameter checker" pointed out to possibility
of the cc708 decoder dereferencing through a null pointer. This code
starts with a default buffer size of 512 and then attempts to allocate
a larger buffer when necessary (in this case, if a single subtitle is
more than 512 characters.) If the reallocation fails, the code would
end up storing the nullptr error return into the buffer pointer, and
then dereference it an an attempt to store the new characters. Catch
this rare condition and drop the characters that won't fit into the
existing buffer.

Fixes #13436.

(cherry picked from commit d0356aba3c9394200a86dcab7d1cf17a558c5eab)