PR middle-end/100307 - spurious -Wplacement-new with negative pointer offset

gcc/ChangeLog:

	PR middle-end/100307
	* builtins.c (compute_objsize_r): Clear base0 for pointers.

gcc/testsuite/ChangeLog:

	PR middle-end/100307
	* g++.dg/warn/Wplacement-new-size-9.C: New test.
	* gcc.dg/tree-ssa/builtin-sprintf-warn-26.c: New test.