xgettext: In language Vala, avoid stack overflow.
* gettext-tools/src/x-vala.c (MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(extract_balanced): Increase and check nesting_depth before calling
extract_balanced recursively.
(extract_vala): Initialize nesting_depth.
* gettext-tools/tests/xgettext-vala-stackovfl-1: New file.
* gettext-tools/tests/xgettext-vala-stackovfl-2: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
