compat/mkdtemp.c: patch for gitmkdtemp() compatibility implementation

Dear all,

I noticed that the current implementation of mkdtemp() in the compat/ directory does not seem to be correct.

gitmkdtemp() is needed for platforms such as Solaris 9 and 10 which do not implement mkdtemp().

The current implementation (compat/mkdtemp.c):

----
char *gitmkdtemp(char *template)
{
        if (!mktemp(template) || mkdir(template, 0700))
                return NULL;
        return template;
}
----

seems to be testing the return value of mktemp incorrectly.

mktemp() always returns the argument 'template', even on failure.

This is an extract from Solaris's mktemp(2) manual page:

RETURN VALUES
     The mktemp() function will  assign  to  template  the  empty
     string if it  cannot create a unique name.

Upon failure, 'template' is set to an empty string, i.e., it's first character is '\0'.

The fixed implementation should read:

----
char *gitmkdtemp(char *template)
{
        if (!*mktemp(template) || mkdir(template, 0700))
                return NULL;
        return template;
}
----

I.e., simply add the pointer dereference operator when calling mktemp().

I have attached a patch against the most current git repo, branch master.

Please let me know if you need the patch in any other way.