Fix clang warning.

Fix clang warning.

The return type of CollectTarget() is Node, so we should return NULL in the
failure case instead of false.

src/ninja.cc:188:16: warning: initialization of pointer of type 'Node *' to null from a constant boolean expression [-Wbool-conversion]
        return false;
               ^~~~~

Signed-off-by: Thiago Farina <tfarina@chromium.org>