Up until now, using NULL as key in av_dict_get() on a non-empty
AVDictionary would crash; using NULL as key in av_dict_set()
would also crash for a non-empty AVDictionary unless AV_DICT_MULTIKEY
was set; in case the dictionary was initially empty or AV_DICT_MULTIKEY
was set, it was even possible for av_dict_set() to succeed when
adding a NULL key, namely when one uses a value != NULL and
the AV_DICT_DONT_STRDUP_VAL flag. Using av_dict_get() on such
an AVDictionary will usually lead to crashes, though.

Fix this by actually checking for key in both functions; error out
if they are NULL.

While just at it, also stop relying on av_strdup(NULL) to return NULL
in av_dict_set().
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>