[RFA/JDWP] Fix ReferenceType.sourceFile

[Kyle Galloway]

Currently, since it is possible to get null returned from 
_Jv_GetInterpClasssourceFile if the class in either native of if the 
information is not available, a call to ReferenceType.sourceFile may 
cause a NPE in writeString which will crash the communication thread 
causing the VM to stop responding to commands.  This patch fixes this by 
checking in VMVirtualMachine::getSourceFile to see if there source file 
is null, and throwing an AbsentInformationException.  Though there are 
two possible causes for this in our implementation, the JDWP spec does 
not distinguish and only allows for an ABSENT_INFORMATION error in this 
case.

ChangeLog
2007-04-11  Kyle Galloway  <kgallowa@redhat.com>

    * gnu/classpath/jdwp/natVMVirtualMachine.cc (getSourceFile): Check
    for null source file and throw an exception indicating this.

Questions/comments/concerns?

Thanks,

Kyle