Cope with NULL dw_cfi_cfa_loc

Cope with NULL dw_cfi_cfa_loc

In def_cfa_0, we may set the 2nd operand's dw_cfi_cfa_loc to NULL, but
then cfi_oprnd_equal_p calls cfa_equal_p with a NULL dw_cfa_location*.
This patch aranges for us to tolerate NULL dw_cfi_cfa_loc.

for  gcc/ChangeLog

	PR middle-end/104540
	* dwarf2cfi.cc (cfi_oprnd_equal_p): Cope with NULL
	dw_cfi_cfa_loc.

for  gcc/testsuite/ChangeLog

	PR middle-end/104540
	* g++.dg/pr104540.C: New.