Remove coding system from UserComment sequence only if it is valid (#147)

Co-authored-by: Grzegorz Ruciński <grucin@mobile4.pro>

Author: grucin

exifread/utils.py

@@ -45,7 +45,9 @@ def make_string_uc(seq) -> str:
     First 8 bytes gives coding system e.g. ASCII vs. JIS vs Unicode.
     """
     if not isinstance(seq, str):
-        seq = seq[8:]
+        # Remove code from sequence only if it is valid
+        if make_string(seq[:8]).upper() in ('ASCII', 'UNICODE', 'JIS', ''):
+            seq = seq[8:]
     # Of course, this is only correct if ASCII, and the standard explicitly
     # allows JIS and Unicode.
     return make_string(seq)