xgettext: In language C, report out-of-range hexadecimal escapes.
This avoids a later crash
"Assertion `UNICODE_VALUE (c) >= 0 && UNICODE_VALUE (c) < 0x110000' failed."
* gettext-tools/src/x-c.c (phase7_getc): When the value of a hexadecimal escape
sequence overflows, warn and use the last value before it overflowed.
* gettext-tools/tests/xgettext-c-7: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
