[Tracker] PATCH: Fix segfault in mp3 extractor when missing the CP1252 code page

[Jonatan Pålsson <jonatan palsson pelagicore com>]

Hi list,

I have two more patches which may be of interest. I will submit them
in separate emails to keep the discussion threads separate.

First off, I noticed an issue in the mp3 extractor when running on a
system with very few code pages installed. It seems that g_convert
does not set an error when a code page is missing, instead it simply
does not convert any bytes. In the function called
"convert_to_encoding" of tracker-extract-mp3.c (for quick reference,
see here: 
https://github.com/Pelagicore/tracker-ivi/blob/mainline_compat/src/tracker-extract/tracker-extract-mp3.c#L693),
this means we may return NULL to id3v2_text_to_utf8 and
id3v24_text_to_utf8 - and pass NULL along from these function as well.

This patch guards the return values of these last two functions from
NULLs when parsing tags. If a null is encountered, the tag can't be
parsed and parsing is skipped.

The old behavior led to a segmentation fault on my machine. To
reproduce the segmentation fault, you could either uninstall the
CP1252 code page, or try returning NULL from these functions.

Patch is here: https://github.com/Pelagicore/tracker-ivi/commit/0f3236f2180d77d1e56a02717ecec95675fd1c5d

--
Regards,
Jonatan Pålsson

Pelagicore AB
Ekelundsgatan 4, 6th floor, SE-411 18 Gothenburg, Sweden