index: shut up warning on uninitialized variable

Even though the `entry` variable will always be initialized when
`read_entry` returns success and even though we never dereference
`entry` in case `read_entry` fails, GCC prints a warning about
uninitialized use. Just initialize the pointer to `NULL` in order to
shut GCC up.

src/index.c

@@ -2494,7 +2494,7 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 
 	/* Parse all the entries */
 	for (i = 0; i < header.entry_count && buffer_size > INDEX_FOOTER_SIZE; ++i) {
-		git_index_entry *entry;
+		git_index_entry *entry = NULL;
 		size_t entry_size = read_entry(&entry, index, buffer, buffer_size, last);
 
 		/* 0 bytes read means an object corruption */