GH-1534: Fix excessive coredump for duplicate enum definitions

An adequate error message was previously reported for duplicate enum
definitions, this just now prevents trying to access it as a constant in
subsequent parsing and further generating a coredump.

testing/btest/language/enum-name-conflict.zeek

@@ -40,3 +40,10 @@ global NOPE = 37;
 redef enum a += {
 	NOPE,
 };
+
+type E: enum { Red, Green, Blue };
+redef enum E += { Pink };
+redef enum E += { Pink };
+
+print Pink;
+print Pink;