Fix erroneous -Werror=missing-braces on old GCC

The buildfarm reports that this is an error on gcc (Debian 4.7.2-5)
4.7.2, 32-bit. The bug seems to be GCC bug 53119, which has obviously
been fixed for years.

Author: Tristan Partin <tristan@neon.tech>
Discussion: https://www.postgresql.org/message-id/flat/CT6HJ3U8068R.3A8SJMV02D9BC@gonk

src/bin/pgbench/pgbench.c

@@ -2239,10 +2239,15 @@ evalStandardFunc(CState *st,
 {
 	/* evaluate all function arguments */
 	int			nargs = 0;
-	PgBenchValue vargs[MAX_FARGS] = { 0 };
 	PgBenchExprLink *l = args;
 	bool		has_null = false;
 
+	/*
+	 * This value is double braced to workaround GCC bug 53119, which seems to
+	 * exist at least on gcc (Debian 4.7.2-5) 4.7.2, 32-bit.
+	 */
+	PgBenchValue vargs[MAX_FARGS] = { { 0 } };
+
 	for (nargs = 0; nargs < MAX_FARGS && l != NULL; nargs++, l = l->next)
 	{
 		if (!evaluateExpr(st, l->expr, &vargs[nargs]))