ovn-trace: Fix pop action in trace.

The pop action execution in trace may print garbage data due to
uninitialized buffer. For example, when it is supposed to print:

pop(reg2) <- 0x50;

it could print:

pop(reg2) <- 0xf8a720000000000060f9000000000000306d572fff7f000032fc466a30560000e06d572fff7f0000f831461c5d7f0000506d572fff7f0000fcf2476a305600001800000000000000f831461c5d7f0000906d572fff7f000079fa476a305600000000000000000000e06d572fff7f00001800000000000000f831461c00000050;

The actual value is the at the 0x50 at the tail but it prints the whole
128 byte buffer. This patch fixes it.

Fixes: 8ce8477 ("actions: Add stack push and pop actions.")
Signed-off-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Ales Musil <amusil@redhat.com>
(cherry picked from commit 543631a)

Author: hzhou8

utilities/ovn-trace.c

@@ -1609,12 +1609,8 @@ execute_pop(const struct ovnact_push_pop *p, struct ofpbuf *stack,
     const void *src = nx_stack_pop(stack, &src_bytes);
     if (src) {
         union mf_subvalue sv;
-        uint8_t dst_bytes = DIV_ROUND_UP(sf.n_bits, 8);
+        memset(&sv, 0, sizeof sv);
 
-        if (src_bytes < dst_bytes) {
-            memset(&sv.u8[sizeof sv - dst_bytes], 0,
-                   dst_bytes - src_bytes);
-        }
         memcpy(&sv.u8[sizeof sv - src_bytes], src, src_bytes);
         mf_write_subfield_flow(&sf, &sv, uflow);
         mf_format_subvalue(&sv, &s);