fortran: Restore interface to its previous state on error [PR48776]

fortran: Restore interface to its previous state on error [PR48776]

Keep memory of the content of the current interface body being parsed
and restore it to its previous state if it has been modified at the time
a parse attempt fails.

This fixes memory errors and random segmentation faults caused by
dangling symbol pointers kept in interfaces' linked lists of symbols.
If a parsing attempt fails and symbols are freed, they should also be
removed from the current interface linked list.

As the list of symbol is a linked list, and parsing only adds new
symbols to the head of the list, all that is needed to track the
previous content of the list is a pointer to its previous head.
This adds such a pointer, and the restoration of the list of symbols
to that pointer on error.

	PR fortran/48776

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_drop_interface_elements_before): New prototype.
	(gfc_current_interface_head): Return a reference to the pointer.
	* interface.cc (gfc_current_interface_head): Ditto.
	(free_interface_elements_until): New function, generalizing
	gfc_free_interface.
	(gfc_free_interface): Use free_interface_elements_until.
	(gfc_drop_interface_elements_before): New function.
	* parse.cc
	(current_interface_ptr, previous_interface_head): New static variables.
	(current_interface_valid_p, get_current_interface_ptr): New functions.
	(decode_statement): Initialize previous_interface_head.
	(reject_statement): Restore current interface pointer to point to
	previous_interface_head.

gcc/testsuite/ChangeLog:

	* gfortran.dg/interface_procedure_1.f90: New test.