xgettext: Fix crash with *.po file input

xgettext: Fix crash with *.po file input

When xgettext was given two *.po files with the same msgid_plural, it
crashed with double-free.  Problem reported by Davlet Panech in:
http://lists.gnu.org/archive/html/bug-gettext/2016-09/msg00001.html
* gettext-tools/src/po-gram-gen.y: Don't free msgid_pluralform after
calling do_callback_message, assuming that it takes ownership.
* gettext-tools/src/read-catalog.c (default_add_message): Free
msgid_plural after calling message_alloc.
* gettext-tools/tests/xgettext-po-2: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add new test.