popen: fix 'may be clobbered' compiler warning

I guess a compiler assumes that the code inside vfork may use the stack
slot that corresponds the variable and so clobber it. The recent commit
07a07b3c ('popen: decouple logger fd
from stderr') adds read from this variable after vfork() in the parent
process.

The warning is produced on RelWithDebInfo build with LTO enabled on GCC
9.2.0 (locally) and on GCC 8.3.0 (in CI).

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/lib/core/popen.c

@@ -784,10 +784,11 @@ struct popen_handle *
 popen_new(struct popen_opts *opts)
 {
 	/*
-	 * Without volatile compiler claims that
-	 * handle might be clobbered from vfork.
+	 * Without volatile compiler claims that those
+	 * variables might be clobbered from vfork.
 	 */
 	struct popen_handle * volatile handle;
+	int volatile log_fd = -1;
 
 	int pfd[POPEN_FLAG_FD_STDEND_BIT][2] = {
 		{-1, -1}, {-1, -1}, {-1, -1},
@@ -853,7 +854,6 @@ popen_new(struct popen_opts *opts)
 	 * reached a file descriptor limit.
 	 */
 	int old_log_fd = log_get_fd();
-	int log_fd = -1;
 	if (old_log_fd >= 0) {
 		log_fd = dup_not_std_streams(old_log_fd);
 		if (log_fd < 0)