Fortran: fix resolution of user-defined operators in named BLOCK [PR126127]

	PR fortran/126127

gcc/fortran/ChangeLog:

	* interface.cc (find_symtree0): Check for NULL pointer.
	(gfc_find_sym_in_symtree): Also search user-defined operator list
	when the symbol refers to a function.

gcc/testsuite/ChangeLog:

	* gfortran.dg/block_18.f90: New test.