The fix was developed in-house and submitted upstream. The upstream bug
is:

http://bugs.proftpd.org/show_bug.cgi?id=4157

diff --git a/modules/mod_core.c b/modules/mod_core.c
--- a/modules/mod_core.c
+++ b/modules/mod_core.c
@@ -40,6 +40,7 @@
 /* From src/main.c */
 extern unsigned long max_connects;
 extern unsigned int max_connect_interval;
+extern unsigned char tracing_enabled;
 
 /* From modules/mod_site.c */
 extern modret_t *site_dispatch(cmd_rec*);
@@ -1381,6 +1382,8 @@
   int per_session = FALSE;
   unsigned int idx = 1;
 
+  tracing_enabled = TRUE;
+
   if (cmd->argc-1 < 1)
     CONF_ERROR(cmd, "wrong number of parameters");
   CHECK_CONF(cmd, CONF_ROOT);
diff --git a/modules/mod_ls.c b/modules/mod_ls.c
--- a/modules/mod_ls.c
+++ b/modules/mod_ls.c
@@ -307,10 +307,12 @@
 
 static int sendline(int flags, char *fmt, ...) {
   va_list msg;
-  char buf[PR_TUNABLE_BUFFER_SIZE+1] = {'\0'};
+  char buf[PR_TUNABLE_BUFFER_SIZE+1];
   int res = 0;
   size_t buflen, listbuflen;
 
+  (void) memset(buf, '\0', sizeof(buf));
+
   if (listbuf == NULL) {
     listbufsz = pr_config_get_server_xfer_bufsz(PR_NETIO_IO_WR);
     listbuf = listbuf_ptr = pcalloc(session.pool, listbufsz);
diff --git a/src/fsio.c b/src/fsio.c
--- a/src/fsio.c
+++ b/src/fsio.c
@@ -556,10 +556,12 @@
 static int cache_stat(pr_fs_t *fs, const char *path, struct stat *sbuf,
     unsigned int op) {
   int res = -1;
-  char pathbuf[PR_TUNABLE_PATH_MAX + 1] = {'\0'};
+  char pathbuf[PR_TUNABLE_PATH_MAX + 1];
   int (*mystat)(pr_fs_t *, const char *, struct stat *) = NULL;
   size_t pathlen;
 
+  (void) memset(pathbuf, '\0', sizeof(pathbuf));
+
   /* Sanity checks */
   if (fs == NULL) {
     errno = EINVAL;
@@ -641,8 +643,8 @@
  * during the hit.
  */
 static pr_fs_t *lookup_dir_fs(const char *path, int op) {
-  char buf[PR_TUNABLE_PATH_MAX + 1] = {'\0'};
-  char tmp_path[PR_TUNABLE_PATH_MAX + 1] = {'\0'};
+  char buf[PR_TUNABLE_PATH_MAX + 1];
+  char tmp_path[PR_TUNABLE_PATH_MAX + 1];
   pr_fs_t *fs = NULL;
   int exact = FALSE;
   size_t tmp_pathlen = 0;
@@ -651,6 +653,9 @@
   pr_fs_match_t *fsm = NULL;
 #endif /* PR_FS_MATCH */
 
+  (void) memset(buf, '\0', sizeof(buf));
+  (void) memset(tmp_path, '\0', sizeof(tmp_path));
+
   sstrncpy(buf, path, sizeof(buf));
 
   /* Check if the given path is an absolute path.  Since there may be
diff --git a/src/trace.c b/src/trace.c
--- a/src/trace.c
+++ b/src/trace.c
@@ -32,6 +32,8 @@
 
 #ifdef PR_USE_TRACE
 
+unsigned char tracing_enabled = FALSE;
+
 static int trace_logfd = -1;
 static unsigned long trace_opts = PR_TRACE_OPT_DEFAULT;
 static pool *trace_pool = NULL;
@@ -473,6 +475,10 @@
   int res;
   va_list msg;
 
+  /* Optimization: do not run tracing code unless explicitly enabled. */
+  if (tracing_enabled == FALSE)
+    return 0;
+
   va_start(msg, fmt);
   res = pr_trace_vmsg(channel, level, fmt, msg);
   va_end(msg);