psql: Fix line continuation prompts for unbalanced parentheses

This was broken by a silly mistake in
e717a9a1.
Reported-by: Jeff Janes <jeff.janes@gmail.com>
Author: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://www.postgresql.org/message-id/CAMkU=1zKGWEJdBbYKw7Tn7cJmYR_UjgdcXTPDqJj=dNwCETBCQ@mail.gmail.com

src/fe_utils/psqlscan.l

@@ -1106,7 +1106,7 @@ psql_scan(PsqlScanState state,
 						result = PSCAN_INCOMPLETE;
 						*prompt = PROMPT_PAREN;
 					}
-					if (state->begin_depth > 0)
+					else if (state->begin_depth > 0)
 					{
 						result = PSCAN_INCOMPLETE;
 						*prompt = PROMPT_CONTINUE;