libsepol/cil: Verify alias in aliasactual statement is really an alias

Nicolas Iooss found while fuzzing secilc with AFL that the statement
"(sensitivityaliasactual SENS SENS)" will cause a segfault.

The segfault occurs because when the aliasactual is resolved the first
identifier is assumed to refer to an alias structure, but it is not.

Add a check to verify that the datum retrieved is actually an alias
and exit with an error if it is not.

Signed-off-by: James Carter <jwcart2@tycho.nsa.gov>
1 file changed