Fix IndexError in GitConfigParser when config value ends in new line

Improve the guarding `if` check in `GitConfigParser`'s `string_decode`
function to safely handle empty strings and prevent `IndexError`s when
accessing string elements.

This resolves an IndexError in the `GitConfigParser`'s `.read()`
method when the config file contains a quoted value containing a
trailing new line.

Fixes:
#1887

fuzzing/fuzz-targets/fuzz_config.py

@@ -35,11 +35,7 @@ def TestOneInput(data):
     except (MissingSectionHeaderError, ParsingError, UnicodeDecodeError):
         return -1  # Reject inputs raising expected exceptions
     except (IndexError, ValueError) as e:
-        if isinstance(e, IndexError) and "string index out of range" in str(e):
-            # Known possibility that might be patched
-            # See: https://github.com/gitpython-developers/GitPython/issues/1887
-            pass
-        elif isinstance(e, ValueError) and "embedded null byte" in str(e):
+        if isinstance(e, ValueError) and "embedded null byte" in str(e):
             # The `os.path.expanduser` function, which does not accept strings
             # containing null bytes might raise this.
             return -1

git/config.py

@@ -452,7 +452,7 @@ def _read(self, fp: Union[BufferedReader, IO[bytes]], fpname: str) -> None:
         e = None  # None, or an exception.
 
         def string_decode(v: str) -> str:
-            if v[-1] == "\\":
+            if v and v[-1] == "\\":
                 v = v[:-1]
             # END cut trailing escapes to prevent decode error
 

test/test_config.py

@@ -142,6 +142,14 @@ def test_multi_line_config(self):
             )
             self.assertEqual(len(config.sections()), 23)
 
+    def test_config_value_with_trailing_new_line(self):
+        config_content = b'[section-header]\nkey:"value\n"'
+        config_file = io.BytesIO(config_content)
+        config_file.name = "multiline_value.config"
+
+        git_config = GitConfigParser(config_file)
+        git_config.read()  # This should not throw an exception
+
     def test_base(self):
         path_repo = fixture_path("git_config")
         path_global = fixture_path("git_config_global")