util.c: check if return value from ttyname_r is > 0 instead of != 0

We must return a negative error code from getttyname_malloc but
that would not be the case if ttyname_r returned a negative value.

ttyname_r should only return EBADF, ENOTTY, or ERANGE so it should
be safe to change.

src/shared/util.c

@@ -2539,7 +2539,7 @@ int getttyname_malloc(int fd, char **r) {
         assert(r);
 
         k = ttyname_r(fd, path, sizeof(path));
-        if (k != 0)
+        if (k > 0)
                 return -k;
 
         char_array_0(path);