fixed substring error when the JAR entry is the WEB_JAR_PREFIX

As discussed, getting the stack trace:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
	at java.lang.String.substring(String.java:1911)
	at bathe.BatheBooter.determineJarOffsets(BatheBooter.java:250)
	at bathe.BatheBooter.run(BatheBooter.java:112)
	at bathe.BatheBooter.main(BatheBooter.java:103)

Debugging indicates that some zip implementations were including WEB_JAR_PREFIX as an entry, which was not expected.