tests: only update last_byte when at the edge

The only possible change of last_byte is when it reaches the edge.
Setting it every time might let last_byte contain an invalid data when
memory corruption is detected, then the check of the next byte will be
incorrect.  For example, a single page corruption at address 0x14ad000
will also lead to a "fake" corruption at 0x14ae000:

  Memory content inconsistency at 14ad000 first_byte = 44 last_byte = 44 current = ef hit_edge = 0
  Memory content inconsistency at 14ae000 first_byte = 44 last_byte = ef current = 44 hit_edge = 0

After the patch, it'll only report the corrputed page:

  Memory content inconsistency at 14ad000 first_byte = 44 last_byte = 44 current = ef hit_edge = 0
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180723123305.24792-4-peterx@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

tests/migration-test.c

@@ -300,6 +300,7 @@ static void check_guests_ram(QTestState *who)
                  * to us yet.
                  */
                 hit_edge = true;
+                last_byte = b;
             } else {
                 fprintf(stderr, "Memory content inconsistency at %x"
                                 " first_byte = %x last_byte = %x current = %x"
@@ -308,7 +309,6 @@ static void check_guests_ram(QTestState *who)
                 bad = true;
             }
         }
-        last_byte = b;
     }
     g_assert_false(bad);
 }