ofproto-dpif-ipfix: Fix inefficent memory use in ipfix_send_template_msgs().

ofproto-dpif-ipfix: Fix inefficent memory use in ipfix_send_template_msgs().

This fixes inefficient use of memory in ipfix_send_template_msgs().
This function was setting up a buffer with a stub, but only the first 4
or 8 bytes of the stub were actually used because the "sizeof" call used
to size it was actually getting the size of a pointer.  This meant that
every template message was causing a series of allocations and
reallocations.

This commit fixes the problem.

Found by Coverity.

Reported-at: https://scan3.coverity.com/reports.htm#v16889/p10449/fileInstanceId=14762995&defectInstanceId=4304799&mergedDefectId=180398
Signed-off-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Justin Pettit <jpettit@ovn.org>