RFR: 8186163: bad signatures should be detected by MetaAccessProvider.parseMethodDescriptor

[Doug Simon]

Please review this small change to improve error handling when an invalid string is passed to MetaAccessProvider.parseMethodDescriptor

https://bugs.openjdk.java.net/browse/JDK-8186163
http://cr.openjdk.java.net/~dnsimon/8186163/

-Doug