Avoid potentially dereferencing a NULL pointer

Avoid potentially dereferencing a NULL pointer

GMIME_IS_MULTIPART and GMIME_IS_MESSAGE both handle NULL pointers
gracefully, but the G_OBJECT_TYPE used in the error handling block
dereferences it without checking it first.

Fix this by checking whether parent->part is valid.

Found using the clang static analyzer.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>