c-family: ICE with -Wconversion and A ?: B [PR101030]

c-family: ICE with -Wconversion and A ?: B [PR101030]

This patch fixes a crash in conversion_warning on a null expression.
It is null because the testcase uses the GNU A ?: B extension.  We
could also use op0 instead of op1 in this case, but it doesn't seem
to be necessary.

	PR c++/101030

gcc/c-family/ChangeLog:

	* c-warn.cc (conversion_warning) <case COND_EXPR>: Don't call
	conversion_warning when OP1 is null.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/cond5.C: New test.