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Per the man page, pcre_study can return NULL without error if
it could not find any additional information.  Errors are indicated
by the combination of a NULL return value and a non-NULL error string.
Fix the handling so that we do not incorrectly reject file_contexts
entries.

Change-Id: I2e7b7e01d85d96dd7fe78545d3ee3834281c4eba
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>