RFR: JDK-8207160: ClassReader::adjustMethodParams can potentially return null if the args list is empty

[Vicente Romero]

Please review the fix for [1] at [2]. The bug was found by a user that 
was compiling some classes with JDK11 while having some old libs built 
with JDK6 in the class path. It seems like some versions of javac6 
produced constructors for inner classes with no arguments. This is not 
expected by newer versions of javac and it was producing a NPE. This fix 
aims at making sure that ClassReader::adjustMethodParams wont return a 
null even if the list of arguments passed to it is empty.

TIA,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8207160
[2] http://cr.openjdk.java.net/~vromero/8207160/webrev.00/jdk.dev.patch