mini_cluster: avoid SIGSEGV when master fails to start up
mini_cluster: avoid SIGSEGV when master fails to start up

My machine's clock became desynchronized causing MiniCluster-based tests to
SIGSEGV. The root cause appears to be in how we manage the mini_masters_
vector. By resizing it right at the beginning, various code paths later on
may iterate on some number of non-existent masters and dereference empty
shared pointers.

Change-Id: Id1cc262ba83f70d97434fca4c75f76b095db77d6
Reviewed-on: http://gerrit.cloudera.org:8080/4006
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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