fix crash issue when using shadowfile with pretty #17853

[changhoetyng]

• Fix crash issue when using --pretty with --shadow-file

Example Scenario:
a.py

b: bytes

b.py

a: int = ""
b: bytes = 1

output

$ mypy a.py --pretty --shadow-file a.py b.py
a.py:1: error: Incompatible types in assignment (expression has type "str",
variable has type "int")
    a: int = ""
             ^~
a.py:2: error: Incompatible types in assignment (expression has type "int",
variable has type "bytes")
    b: bytes = 1
               ^

Fixes #17853

[JukkaL]

Thanks for the PR!

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅