Use more specific regex chars to prevent ReDoS

* CVE-2021-25292

Author: hugovk

src/PIL/PdfParser.py

@@ -580,8 +580,9 @@ def next_object_id(self, offset=None):
     whitespace_or_hex = br"[\000\011\012\014\015\0400-9a-fA-F]"
     whitespace_optional = whitespace + b"*"
     whitespace_mandatory = whitespace + b"+"
+    whitespace_optional_no_nl = br"[\000\011\014\015\040]*"  # no "\012" aka "\n"
     newline_only = br"[\r\n]+"
-    newline = whitespace_optional + newline_only + whitespace_optional
+    newline = whitespace_optional_no_nl + newline_only + whitespace_optional_no_nl
     re_trailer_end = re.compile(
         whitespace_mandatory
         + br"trailer"