fix enumtostr crash for enum-range (#13035)

tests/enum/tenum_no_rtti.nim

@@ -0,0 +1,12 @@
+discard """
+  output: '''A
+B'''
+  cmd: '''nim c --gc:arc $file'''
+"""
+type
+  Enum = enum A, B, C
+  EnumRange = range[A .. B]
+proc test_a(x: Enum): string = $x
+proc test_b(x: EnumRange): string = $x
+echo test_a(A)
+echo test_b(B)