NPE when rewriting stack trace

[Maurizio Cimadamore]

Hi,
I was experimenting with the Panama memory access API in jshell and I 
got this:


jshell> arr_elem_h.set(segment.baseAddress(), 4, 3);
|  Exception java.lang.IndexOutOfBoundsException: Out of bound access on 
segment MemorySegment{ id=0x60ec3cd3 limit: 40 }; new offset = 40; new 
length = 8
|        at MemorySegmentImpl.checkBounds (MemorySegmentImpl.java:201)
|        at MemorySegmentImpl.checkRange (MemorySegmentImpl.java:180)
|        at MemoryAddressImpl.checkAccess (MemoryAddressImpl.java:86)
|        at VarHandleMemoryAddressAsDoubles.checkAddress 
(VarHandleMemoryAddressAsDoubles.java:55)
|        at VarHandleMemoryAddressAsDoubles.set0 
(VarHandleMemoryAddressAsDoubles.java:80)
|        at VarHandleMemoryAddressAsDoubles1/0x0000000800b9f440.set 
(Unknown Source)
|        at VarHandleGuards.guard_LII_V (VarHandleGuards.java:719)
|        at (#9:1)


The following patch (discussed with Jan) fixes it:

http://cr.openjdk.java.net/~mcimadamore/panama/8234814_v3/src/jdk.jshell/share/classes/jdk/jshell/Eval.java.udiff.html

For now I've refrained from fixing this in Panama directly - as it will 
probably be better to fix it more generally in the upstream JDK (after 
all, we can have VarHandle or MethodHandle throwing exceptions w/o 
Panama as well).


If you want I can go ahead an file a bug for this

Maurizio