Fix crash in astdiff.py related to ec11a50. (#5126)

This fixes a crash reported in Quip's codebase related to UNBOUND_IMPORTED symbols.
Unfortunately we don't have a nice test case.

Author: whiten

mypy/server/aststrip.py

@@ -213,7 +213,8 @@ def visit_import(self, node: Import) -> None:
             for name, as_name in node.ids:
                 imported_name = as_name or name
                 initial = imported_name.split('.')[0]
-                self.names[initial] = SymbolTableNode(UNBOUND_IMPORTED, None)
+                if initial in self.names:
+                    self.names[initial] = SymbolTableNode(UNBOUND_IMPORTED, None)
 
     def visit_import_all(self, node: ImportAll) -> None:
         # Imports can include both overriding symbols and fresh ones,