Fix a memory leak in dumping functions with TRANSFORMs

Fix a memory leak in dumping functions with TRANSFORMs

The gneration of the dump clause for functions with TRANSFORM
calls would leak the memory for holding the result of the Oid
array parsing.  Fix by freeing.

While in there, switch to using pg_malloc instead of palloc in
order to be consistent with the rest of the file.

Author: Oleg Tselebrovskiy <o.tselebrovskiy@postgrespro.ru>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/baf1ae4511288e5b421f41e79a3df1a0@postgrespro.ru