testsuite: add various -Wanalyzer-null-dereference false +ve test cases

There are various -Wanalyzer-null-dereference false +ves in bugzilla
that I've been attempting to fix.  Unfortunately I haven't made much
progress, but it seems worth at least capturing the reduced
reproducers as test cases, to make it easier to spot changes in
behavior.

gcc/testsuite/ChangeLog:
	PR analyzer/102671
	PR analyzer/105755
	PR analyzer/108251
	PR analyzer/108400
	* gcc.dg/analyzer/null-deref-pr102671-1.c: New test, reduced
	from Emacs.
	* gcc.dg/analyzer/null-deref-pr102671-2.c: Likewise.
	* gcc.dg/analyzer/null-deref-pr105755.c: Likewise.
	* gcc.dg/analyzer/null-deref-pr108251-smp_fetch_ssl_fc_has_early-O2.c:
	New test, reduced from haproxy's src/ssl_sample.c.
	* gcc.dg/analyzer/null-deref-pr108251-smp_fetch_ssl_fc_has_early.c:
	Likewise.
	* gcc.dg/analyzer/null-deref-pr108400-SoftEtherVPN-WebUi.c: New
	test, reduced from SoftEtherVPN's src/Cedar/WebUI.c.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>