Fix out-of-memory handling in ecpglib.

Fix out-of-memory handling in ecpglib.

ecpg_build_params() would crash on a null pointer dereference if
realloc() failed, due to updating the persistent "stmt" struct
too aggressively.  (Even without the crash, this would've leaked
the old storage that we were trying to realloc.)

Per Coverity.  This seems to have been broken in commit 0cc050794,
so back-patch into v12.