Prevent check-local-headers.sh hang.

Consensus in [1] was that the right way to fix this was simply to
redirect from /dev/null to terminate the test if no inputs was found
by the tests own globbing e.g. "*/*.{o,os,oS}.d" This can happen when
the build is incomplete and the tests are started. This failure causes
check-local-headers.sh to hang forever waiting for input from stdin.
This change makes the test terminate immediately if the glob doesn't
return any files.

[1] https://sourceware.org/ml/libc-alpha/2014-12/msg00337.html

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