c++: Fix error recovery after export keyword [PR121832]

c++: Fix error recovery after export keyword [PR121832]

When we enter cp_parser_explicit_template_declaration with the following
tokens being 'template <>', we never parse a parameter list and so with
-fconcepts we crash dereferencing a null pointer.  This can currently
only happen after a non-modules 'export' declaration, as all other paths
check early for this case.

	PR c++/121832

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_explicit_template_declaration): Check for
	null.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/pr121832.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>