patch 8.2.3106: Vim9: confusing line number reported for error

Problem:    Vim9: confusing line number reported for error.
Solution:   Use the start line number for the store instruction.
            (closes #8488)

src/testdir/test_vim9_assign.vim

@@ -388,6 +388,16 @@ def Test_assign_linebreak()
           ->copy()
   END
   CheckDefFailure(lines, 'E1012:', 2)
+
+  lines =<< trim END
+      var x: any
+      x.key = 1
+          + 2
+          + 3
+          + 4
+          + 5
+  END
+  CheckDefExecAndScriptFailure2(lines, 'E1148:', 'E1203:', 2)
 enddef
 
 def Test_assign_index()