RFR(XS): JDK-8217441: Failure of ::realloc() should be handled correctly in sawindbg.cpp

[Alex Menkov]

Hi all,

please review small (and I'd say trivial) fix for
https://bugs.openjdk.java.net/browse/JDK-8217441
webrev:
http://cr.openjdk.java.net/~amenkov/jdk15/WinDbg_realloc/webrev/
from realloc() spec:
On failure, returns a null pointer. The original pointer ptr remains 
valid and may need to be deallocated with free() or realloc().

--alex