Make sure we use initialized variable.

Addressing
pylint-3 --disable=R --disable=C --indent-string="\t" --extension-pkg-whitelist=rpm,lxml lib/*/*.py
************* Module swidtags
lib/dnf-plugins/swidtags.py:312:11: E0606: Possibly using variable 'ts' before assignment (possibly-used-before-assignment)

lib/dnf-plugins/swidtags.py

@@ -308,8 +308,8 @@ def run_rpm2swidtag_for(self, pkgs):
 
 	def get_nevra_checksum(self, nevra, verbose=True):
 		if not self.ts:
-			ts = rpm.transaction.initReadOnlyTransaction(root=self.base.conf.installroot)
-		rpms = ( ts.dbMatch(2, str(nevra)) )
+			self.ts = rpm.transaction.initReadOnlyTransaction(root=self.base.conf.installroot)
+		rpms = ( self.ts.dbMatch(2, str(nevra)) )
 		if len(rpms) > 1:
 			if verbose:
 				logger.warning("Multiple rpms %s found installed for package %s.", str(rpms), str(nevra))