backport from 10.0

mtr can crash occasionally.
This happens when mtr sends to a child mtr process (or vice-versa) a packet,
that gets truncated or, perhaps, split in two.
Then the other side cannot deserialize it and fails as above.

mysql-test/lib/My/Test.pm

@@ -70,7 +70,7 @@ sub write_test {
 
   my $serialized= Storable::freeze($test);
   $serialized =~ s/([\x0d\x0a\\])/sprintf("\\%02x", ord($1))/eg;
-  print $sock $header, "\n", $serialized, "\n";
+  send $sock,$header. "\n". $serialized. "\n", 0;
 }