soup_form_decode_multipart: check that msg really is a non-NULL message
soup_form_decode_multipart: check that msg really is a non-NULL message

If it wasn't, the next line would segfault anyway.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=669479
Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Reviewed-by: Dan Winship <a4113c7b8a2a3e6b96e702b23e93a123ab762619@gnome.org>

jwendell/libsoup,Distrotech/libsoup,fanc999/libsoup,jwendell/libsoup,Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup,fanc999/libsoup,jwendell/libsoup,fanc999/libsoup,Distrotech/libsoup,jwendell/libsoup,fanc999/libsoup,Distrotech/libsoup,jwendell/libsoup