fix memory leak in resultproxy.c

fix memory leak in resultproxy.c

the error raised for non-existent attribute didn't free
the "name" string, causing a leak.

Fixed a memory leak in the C extensions which could occur when calling upon
named members of :class:`.Row` when the member does not exist under Python
3; in particular this could occur during numpy transformations when it
attempts to call members such as ``.__array__``, but the issue was
surrounding any ``AttributeError`` thrown by the :class:`.Row` object. This
issue does not apply to version 2.0 which has already transitioned to
Cython. Thanks much to Sebastian Berg for identifying the problem.

Fixes: #7875
Change-Id: I444026a877ea1473a5ffac592c7f36ed6f4b563e