fixes potential memory leak static analyzer warning

clang static analyzer was outputting this warning, this commit addresses the problem:

JTMappings.m:31:9: Potential leak of an object
JTMappings.m:30:23: Method returns an Objective-C object with a +1 retain count
JTMappings.m:31:9: Object leaked: allocated object is not referenced later in this execution path and has a retain count of +1