natVMVirtualMachine.cc (getSourceFile): Check for null source file and throw an exception indicating this.

2007-04-23  Kyle Galloway  <kgallowa@redhat.com>

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

From-SVN: r124066