Fix crash on invalid star lvalue (#22222)

Fixes #20796

I think the correct fix is to not create any placeholders in the first
place, since this is essentially a syntax error.

Author: ilevkivskyi

mypy/semanal.py

@@ -8496,16 +8496,16 @@ def names_modified_by_assignment(s: AssignmentStmt) -> list[NameExpr]:
     return result
 
 
-def names_modified_in_lvalue(lvalue: Lvalue) -> list[NameExpr]:
+def names_modified_in_lvalue(lvalue: Lvalue, include_star: bool = False) -> list[NameExpr]:
     """Return all NameExpr assignment targets in an Lvalue."""
     if isinstance(lvalue, NameExpr):
         return [lvalue]
-    elif isinstance(lvalue, StarExpr):
+    elif isinstance(lvalue, StarExpr) and include_star:
         return names_modified_in_lvalue(lvalue.expr)
     elif isinstance(lvalue, (ListExpr, TupleExpr)):
         result: list[NameExpr] = []
         for item in lvalue.items:
-            result += names_modified_in_lvalue(item)
+            result += names_modified_in_lvalue(item, include_star=True)
         return result
     return []
 

test-data/unit/check-incremental.test

@@ -8293,3 +8293,15 @@ Jérôme = False
 [stale]
 [out]
 [out2]
+
+[case testInvalidUnpackTargetFromUndefinedNoCrash]
+*a = b
+a
+[out]
+main:1: error: Starred assignment target must be in a list or tuple
+main:1: error: Name "b" is not defined
+main:2: error: Name "a" is not defined
+[out2]
+main:1: error: Starred assignment target must be in a list or tuple
+main:1: error: Name "b" is not defined
+main:2: error: Name "a" is not defined