RFR: 8225715: jhsdb jmap fails to write binary heap dump of a jshell process

[Fairoz Matte]

Hi,

Please review a fix that potentially handle NPE and allow heap to dump from a 
process were we get source file name as null.

Background: 
For taking heap dump of JShell process, we get getSourceFileName() as null. 
Current implementation doesn't have null check.
This fix will handle the null check and avoid breaking in call to writeObjectID(sym).
Regression test provided with patch will demonstrate the problem.

JBS bug - https://bugs.openjdk.java.net/browse/JDK-8225715
Webrev - http://cr.openjdk.java.net/~fmatte/8225715/webrev.00/

Testing: Mach5 tier1,tier2 tier3, hs-tier1, hs-tier2 and hs-tier3 tests successfully passed

Thanks,
Fairoz