[PATCH v3] builtin/mv.c: fix possible segfault in add_slash()

A possible segfault was introduced in c08830de41 (mv: check if <destination> is a SKIP_WORKTREE_DIR, 2022-08-09).

When running t7001 with SANITIZE=address, problem appears when running:

	git mv path1/path2/ .
or
	git mv directory ../
or
	any <destination> that makes dest_path[0] an empty string.

The add_slash() call could segfault when path argument to it is an empty string, because it makes an out-of-bounds read to decide if an extra slash '/' needs to be appended to it.

As add_slash() is used to make sure that a valid pathname to a file in the given directory can be made by appending a filename after the value returned from it, if path is an empty string, we want to return it as-is. The path to a file "F" in the top-level of the working tree (i.e. path=="") is formed by appending "F" after "" (i.e. path) without any slash in between.

So, just like the case where a non-empty path already ends with a slash, return an empty path as-is.

Reported-by: Jeff King <peff@peff.net>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
---
Range-diff against v2:
1:  1120dc7e6b ! 1:  569e618013 builtin/mv.c: fix possible segfault in add_slash()
    @@ Commit message
         or
                 any <destination> that makes dest_path[0] an empty string.
     
    -    The add_slash() call could segfault when dest_path[0] is an empty string,
    -    because it was accessing a null value in such case.
    -
    -    Change add_slash() to check the path argument is a non-empty string
    -    before accessing its value. If the path is empty, return it as-is.
    -
    -    Explanation:
    -
    -    It's OK for add_slash() to return an empty string as-is. add_slash()
    -    converts its path argument to the prefix (for "folder1/file1",
    -    "folder1/" is the prefix we mean here) for the result path. The path
    -    argument is an empty string _iff_ the result path is analyzed to be at
    -    the top level (this normalization process is done earlier by
    -    internal_prefix_pathspec()).
    -
    -    Because the prefix for a top-level path is an empty string, thus
    -    add_slash() should return an empty path argument as-is, both for
    -    correctness and avoiding inappropriate memory access.
    +    The add_slash() call could segfault when path argument to it is an empty
    +    string, because it makes an out-of-bounds read to decide if an extra
    +    slash '/' needs to be appended to it.
    +
    +    As add_slash() is used to make sure that a valid pathname to a file in
    +    the given directory can be made by appending a filename after the value
    +    returned from it, if path is an empty string, we want to return it
    +    as-is.  The path to a file "F" in the top-level of the working tree
    +    (i.e. path=="") is formed by appending "F" after "" (i.e. path) without
    +    any slash in between.
    +
    +    So, just like the case where a non-empty path already ends with a slash,
    +    return an empty path as-is.
     
         Reported-by: Jeff King <peff@peff.net>
         Helped-by: Jeff King <peff@peff.net>

 builtin/mv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)