javacomp: Silence -Wanalyzer-possible-null-argument warning.

javacomp: Silence -Wanalyzer-possible-null-argument warning.

* lib/javacomp.c: Include verify.h.
(is_envjavac_gcj43_usable, is_envjavac_oldgcj_14_13_usable,
is_envjavac_nongcj_usable, compile_java_class): Assert that the
xasprintf results are non-NULL. This is possible since all involved
format strings are valid and don't use %ls, and all argument strings
are small compared to INT_MAX.
* modules/javacomp (Depends-on): Add verify.