g_strcompress: check that source is non-NULL rather than just crashing

g_strcompress: check that source is non-NULL rather than just crashing

Calling this function with a NULL argument is considered to be invalid,
but one of the regression tests does it anyway (to watch it crash), which
seems a good indication that it's expected to be somewhat common.
Let's check it rather than segfaulting.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=666113
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>
Reviewed-by: Matthias Clasen <mclasen@redhat.com>