ob1: OBJ_CONSTRUCT the convertor in the send_inline optimization.

This change does not appear to increase the small message latency of ping-pong
benchmarks and fixes an issue found by our ibm datatype tests.

Fixes trac:4232

cmr=v1.7.5:ticket=trac:4149

This commit was SVN r30598.

The following Trac tickets were found above:
  Ticket 4149 --> https://svn.open-mpi.org/trac/ompi/ticket/4149
  Ticket 4232 --> https://svn.open-mpi.org/trac/ompi/ticket/4232

ompi/mca/pml/ob1/pml_ob1_isend.c

@@ -80,7 +80,7 @@ static inline int mca_pml_ob1_send_inline (void *buf, size_t count,
 
     if (count > 0) {
         /* initialize just enough of the convertor to avoid a SEGV in opal_convertor_cleanup */
-        convertor.stack_size = 0;
+        OBJ_CONSTRUCT(&convertor, opal_convertor_t);
 
         /* We will create a convertor specialized for the        */
         /* remote architecture and prepared with the datatype.   */