minor performance tweak

jquery.inputmask.js

@@ -214,7 +214,8 @@ This plugin is based on the masked input plugin written by Josh Bush (digitalbus
         }
 
         function isMask(pos) {
-            return tests[determineTestPosition(pos)] ? tests[determineTestPosition(pos)][0] : false;
+            var testPos = determineTestPosition(pos);
+            return tests[testPos] ? tests[testPos][0] : false;
         }
 
         function isFirstMaskOfBlock(testPosition) {