gcontenttype: Fix a potential NULL pointer dereference

gcontenttype: Fix a potential NULL pointer dereference

If the initial part of the header (‘MIME-TreeMagic’) is valid, but the
following line does not start with ‘[’ (i.e. is not a valid section
line), insert_matchlet() will be called with a NULL match pointer, and
will crash with a NULL pointer dereference.

Fix this by bailing out if a valid section line isn’t encountered before
the first insert_matchlet() call (i.e. between the header line and the
first data line).

Note that this has not been tested against a real treemagic file; the
fix is purely theoretical.

Found by scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=113075