index: fix contradicting comparison

The overflow check in `read_reuc` tries to verify if the
`git__strtol32` parses an integer bigger than UINT_MAX. The `tmp`
variable is casted to an unsigned int for this and then checked
for being greater than UINT_MAX, which obviously can never be
true.

Fix this by instead fixing the `mode` field's size in `struct
git_index_reuc_entry` to `uint32_t`. We can now parse the int
with `git__strtol64`, which can never return a value bigger than
`UINT32_MAX`, and additionally checking if the returned value is
smaller than zero.

We do not need to handle overflows explicitly here, as
`git__strtol64` returns an error when the returned value would
overflow.

include/git2/sys/index.h

@@ -25,7 +25,7 @@ typedef struct git_index_name_entry {
 
 /** Representation of a resolve undo entry in the index. */
 typedef struct git_index_reuc_entry {
-	unsigned int mode[3];
+	uint32_t mode[3];
 	git_oid oid[3];
 	char *path;
 } git_index_reuc_entry;