Fix compiler abort during liveness detection.

The compiler would abort during liveness detection on an if-then-else goal
with an unreachable Then branch, due to the Then goal not having its
liveness-related fields initialised.  Bug #51.

compiler/liveness.m:
	    As above.

tests/valid/Mercury.options:
tests/valid/Mmakefile:
tests/valid/bug51.m:
	    Add test case.

NEWS:
	    Announce the change.