RFR: 8246434: Threads::print_on_error assumes that the heap has been set up

[Stefan Karlsson]

Hi all,

Please review this trivial patch to null check the access to the heap, 
when printing threads to the hs_err files.

https://cr.openjdk.java.net/~stefank/8246434/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8246434

Output from an initialization crash without this fix:
---
Other Threads:

[error occurred during error reporting (printing all threads), id 0xb, 
SIGSEGV (0xb) at pc=0x0000000104cbb609]
---

with the fix:
---
Other Threads:

=>0x00007ff10980e220 (exited) JavaThread "Unknown thread" 
[_thread_in_vm, id=7171, stack(0x00007000083dc000,0x00007000084dc000)]
---

Thanks,
StefanK