Check for a NULL pointer, do not call Build.

When calling BacktraceMap::Create(), a NULL pointer is returned if Build
fails. Building twice can cause problems and might leak memory.

(cherry picked from commit 836572a07142627ff291d686a4e9e03a0988344b)

Change-Id: I59d29bb6e5324de6eb099916045c2ab1d9e56630
1 file changed