[BUGFIX] Fix crash due to sizeof bug in RegisterExtensionExplainOption

Hi hackers,

The allocations in src/backend/commands/explain_state.c
used sizeof(char *) instead of sizeof(ExplainExtensionOption),
which could cause a crash if an extension would register
more than 8 extension EXPLAIN options:

Attached small example extension, test_explain_state.txt,
to demonstrate the bug:

LOAD 'test_explain_state';
LOAD
EXPLAIN (test_explain_opt9) SELECT 1;
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
connection to server was lost

/Joel