Fix memory corruption in C++ demangler

The __cxa_demangle_gnu3() and cpp_demangle_gnu3() functions segfault on
various libcxxabi test cases due to a copy and paste error. This change
fixes that.

This is a subset of https://github.com/libcxxrt/libcxxrt/pull/34 which
fixes the immediate problem.

Reviewed by: imp, emaste (I think)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1222