RFR: 8145073: Filename and linenumber are not printed for assers any more.

[David Lindholm]

Hello Runtime,

Please review this patch that fixes the posix version of 
os::exception_name(). The problem is that this function in a recent 
change started returning a string saying "invalid (<num>)" instead of 
NULL for invalid exception numbers, breaking the API. Accoring to os.hpp 
the API is still:

   // returns a string to describe the exception/signal;
   // returns NULL if exception_code is not an OS exception/signal.

.. and this is also what windows does. This change in return value for 
the posix version makes the NULL-checks for calls to this function in 
vmError.cpp invalid, leading to that we don't get filename and 
linenumber printed in the hs_err file for asserts (for example).

Bug: https://bugs.openjdk.java.net/browse/JDK-8145073
Webrev: http://cr.openjdk.java.net/~david/JDK-8145073/webrev.00/

Testing: JPRT.


Thanks,
David