Patch for fixing a few memory leaks

Patch fix memory leaks in src/backend/utils/fmgr/dfmgr.c .
This leaks is very significant with massive update/insert tables with gist
indexes in one transaction or with following sequence of commands:
1. COPY in table large number of row
2. CREATE GiST index on table
3. VACUUM ANALYZE
On third step postgres eats very big number of memory.
This patch fix it.

BTW
Tom, I want to notice that initGISTstate is called for every inserting value
(for each row). I think it's not good, because this function called 'fmgr_info'
7 times. 'fmgr_info' call a 'load_external_function' with execution of sequence
search on library name. Any suggestion?

--
Teodor Sigaev
teodor(at)stack(dot)net