BacktraceBuilder._cprefs needs to be accessed as unsigned short

This is similar to the issue fixed (and hopefully backported to 8) with JDK-8194246. That one fixed the "methods" array that is declared as "short[]", handles "u2" elements, but mistakenly accessed as "short", not "ushort".

There is a similar trouble with "cprefs" array, that is declared as "short[]", handles "u2" elements, and yet again accessed as "short". Note that JDK-8140685 rewired this to short T_LONG/T_INT as the Symbol* address.

This means we have a problem in 8u, but not anywhere else.

This is fairly easy to reproduce on the test case from JDK-8194246, if we simulate redefinition by doing this:

   // The method can be NULL if the requested class version is gone
- Symbol* sym = (method != NULL) ? method->name() : holder->constants()->symbol_at(cpref);
+ Symbol* sym = holder->constants()->symbol_at(cpref);

Then the same out-of-bounds assert fires, now at cpref-taking path.