RFR(S): 8144601: Premature assert in directive inline parsing

[Nils Eliasson]

Hi,

Please review this small change. It fixes a case when the directives 
parsers can trigger an assert when parsing malformed inline commands. 
This assert triggers right before the error handling, so it has always 
run fine in product builds. The fix makes sure we bail out directly.

Testing:
The test case that caught the bug has been verified

Webrev: http://cr.openjdk.java.net/~neliasso/8144601/webrev.02/
Bug: https://bugs.openjdk.java.net/browse/JDK-8144601

Regards,
Nils Eliasson