dynamic-string: Fix a crash in ds_clone().

ds_clone() crashes while trying to clone an empty dynamic string.
It happens because it doesn't check if memory was allocated and
tries to read from the NULL pointer.  ds_init() doesn't allocate
any memory.

For example:
In netdev_offload_dpdk_flow_create() when an offload request fails,
dump_flow() is called to log a warning message. The 's_tnl' string
in flow_patterns gets initialized in vport_to_rte_tunnel() conditionally
via ds_put_format(). If it is not initialized, it crashes later in
dump_flow_attr()->ds_clone()->memcpy() while dereferencing this string.

To fix this, check if memory for the src string has been allocated,
before copying it to the dst string.

Fixes: fa44a4a ("ovn-controller: Persist desired conntrack groups.")
Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/dynamic-string.c

@@ -460,6 +460,10 @@ ds_chomp(struct ds *ds, int c)
 void
 ds_clone(struct ds *dst, struct ds *source)
 {
+    if (!source->allocated) {
+        ds_init(dst);
+        return;
+    }
     dst->length = source->length;
     dst->allocated = dst->length;
     dst->string = xmalloc(dst->allocated + 1);