test/autofs: fix use-after-free

autofs.c:66:17: error: pointer 'str' may be used after 'realloc' [-Werror=use-after-free]

autofs.c: In function 'check_automount':
../lib/zdtmtst.h:131:9: error: pointer 'mountpoint' may be used after 'free' [-Werror=use-after-free]
  131 |         test_msg("ERR: %s:%d: " format " (errno = %d (%s))\n", __FILE__, __LINE__, ##arg, errno, strerror(errno))
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
autofs.c:277:17: note: in expansion of macro 'pr_perror'
  277 |                 pr_perror("%s: failed to close fd %d", mountpoint, p->fd);
      |                 ^~~~~~~~~
autofs.c:268:9: note: call to 'free' here
  268 |         free(mountpoint);
      |         ^~~~~~~~~~~~~~~~

Fixes: #1731

v2: (@Snorch) always update `str` after successful realloc()

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

test/zdtm/static/autofs.c

@@ -47,14 +47,14 @@ static char *xvstrcat(char *str, const char *fmt, va_list args)
 		ret = -ENOMEM;
 		new = realloc(str, offset + delta);
 		if (new) {
+			str = new;
 			va_copy(tmp, args);
 			ret = vsnprintf(new + offset, delta, fmt, tmp);
 			va_end(tmp);
 			if (ret >= delta) {
 				/* NOTE: vsnprintf returns the amount of bytes
 				 *                                  * to allocate. */
 				delta = ret + 1;
-				str = new;
 				ret = 0;
 			}
 		}
@@ -266,6 +266,7 @@ static int check_automount(struct autofs_params *p)
 		return err;
 
 	free(mountpoint);
+	mountpoint = NULL;
 
 	err = p->setup(p);
 	if (err) {
@@ -274,7 +275,7 @@ static int check_automount(struct autofs_params *p)
 	}
 
 	if (close(p->fd)) {
-		pr_perror("%s: failed to close fd %d", mountpoint, p->fd);
+		pr_perror("mountpoint failed to close fd %d", p->fd);
 		return -errno;
 	}