[common] add DCHECK to spot bugs in using Schema

While reviewing recently posted patch [1], I was looking at the code in
src/kudu/common/schema.h and decided to add more DCHECK-like asserts
to help catching bugs in the implementation of the Schema class itself
and spotting errors at the call sites where Schema objects are used.

This patch doesn't contain any functional changes.

[1] https://gerrit.cloudera.org/#/c/18213/

Change-Id: I44b73ef06924af6556a7cd3da4a7eec20d12aefc
Reviewed-on: http://gerrit.cloudera.org:8080/18231
Reviewed-by: Andrew Wong <anjuwong@g.ucla.edu>
Reviewed-by: Mahesh Reddy <mreddy@cloudera.com>
Tested-by: Alexey Serbin <alexey@apache.org>
1 file changed