ovs build: Fix memset with zero size warning.

In file included from /usr/include/string.h:640:0,
                 from ./lib/string.h:20,
                 from /usr/include/netinet/icmp6.h:22,
                 from ../lib/flow.h:21,
                 from ../lib/flow.c:18:
In function 'memset',
    inlined from 'flow_push_vlan_uninit' at ../lib/flow.c:2188:19:
/usr/include/x86_64-linux-gnu/bits/string3.h:81:30: error:
call to '__warn_memset_zero_len' declared with attribute warning:
memset used with constant zero length parameter; this could be
due to transposed parameters [-Werror]
       __warn_memset_zero_len ();
                              ^
cc1: all warnings being treated as errors
make[2]: *** [lib/flow.lo] Error 1

Fixes: f0fb825 ("Add support for 802.1ad (QinQ tunneling)")
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/flow.c

@@ -2222,7 +2222,9 @@ flow_push_vlan_uninit(struct flow *flow, struct flow_wildcards *wc)
 {
     if (wc) {
         int n = flow_count_vlan_headers(flow);
-        memset(wc->masks.vlans, 0xff, sizeof(union flow_vlan_hdr) * n);
+        if (n) {
+            memset(wc->masks.vlans, 0xff, sizeof(union flow_vlan_hdr) * n);
+        }
     }
     memmove(&flow->vlans[1], &flow->vlans[0],
             sizeof(union flow_vlan_hdr) * (FLOW_MAX_VLAN_HEADERS - 1));