Handle null cfun [PR102243].

Resolves:
PR middle-end/102243 - ICE on placement new at global scope

gcc/ChangeLog:
	PR middle-end/102243
	* tree-ssa-strlen.c (get_range): Handle null cfun.

gcc/testsuite/ChangeLog:
	PR middle-end/102243
	* g++.dg/warn/Wplacement-new-size-10.C: New test.