Use explicit_bzero() when clearing sslpassword in libpq

Since 74a308c, any security-sensitive information gets cleared from
memory this way.  This was forgotten in 4dc6355.

Author: Daniel Gustafsson
Reviewed-by: Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/935443BA-D42E-4CE0-B181-1AD79E6DD45A@yesql.se

src/interfaces/libpq/fe-connect.c

@@ -4037,7 +4037,10 @@ freePGconn(PGconn *conn)
 	if (conn->sslkey)
 		free(conn->sslkey);
 	if (conn->sslpassword)
+	{
+		explicit_bzero(conn->sslpassword, strlen(conn->sslpassword));
 		free(conn->sslpassword);
+	}
 	if (conn->sslrootcert)
 		free(conn->sslrootcert);
 	if (conn->sslcrl)