Don't segfault if t_enum::get_constant_by_name fails

The recent enum change was causing t_enum::get_constant_by_name to fail
in t_const_value::get_integer.  This was causing a difficult-to-debug
segfault.  Check for failure and throw an exeception.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@996609 13f79535-47bb-0310-9956-ffa450edef68
1 file changed