account for invalid data in enum $ on arc/orc (#24886)

closes #24875

Refc gives `0 (invalid data!)`, but since enum `$` procs on arc are
generated during enum declarations we might not have access to string
concatenation and integer `$`, so it generates a static string. Just
chose an empty string for this.

(cherry picked from commit 5aaba213d4)

compiler/enumtostr.nim

@@ -33,6 +33,10 @@ proc genEnumToStrProc*(t: PType; info: TLineInfo; g: ModuleGraph; idgen: IdGener
     caseStmt.add newTree(nkOfBranch, newIntTypeNode(field.position, t),
       newTree(nkStmtList, newTree(nkFastAsgn, newSymNode(res), newStrNode(val, info))))
     #newIntTypeNode(nkIntLit, field.position, t)
+  # safety branch for invalid data:
+  caseStmt.add newTree(nkElse,
+    newTree(nkStmtList, newTree(nkFastAsgn, newSymNode(res),
+      newStrNode("", info))))
 
   body.add(caseStmt)
 

tests/arc/tinvalidenumtostr.nim

@@ -0,0 +1,9 @@
+# issue #24875
+
+type
+  MyEnum = enum
+    One = 1
+
+var x = cast[MyEnum](0)
+let s = $x
+doAssert s == ""