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 <328b27c99317b12d4abcd46c4810451f3997abf9@chromium.org>

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