A conditional expression between DFP and non-DFP floating-point
produces an ICE.  This patch fixes this by making
build_conditional_expr return early when c_common_type produces an
error.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

	PR c/71601
gcc/c:
	* c-typeck.c (build_conditional_expr): Return error_mark_node if
	c_common_type returns error_mark_node.

gcc/testsuite:
	* gcc.dg/dfp/usual-arith-conv-bad-3.c: New test.

From-SVN: r237622