Add missing bad-PGconn guards in libpq entry points.

There's a convention that externally-visible libpq functions should
check for a NULL PGconn pointer, and fail gracefully instead of
crashing.  PQflush() and PQisnonblocking() didn't get that memo
though.  Also add a similar check to PQdefaultSSLKeyPassHook_OpenSSL;
while it's not clear that ordinary usage could reach that with a
null conn pointer, it's cheap enough to check, so let's be consistent.

Daniele Varrazzo and Tom Lane

Discussion: https://postgr.es/m/CA+mi_8Zm_mVVyW1iNFgyMd9Oh0Nv8-F+7Y3-BqwMgTMHuo_h2Q@mail.gmail.com

src/interfaces/libpq/fe-exec.c

@@ -3829,6 +3829,8 @@ PQsetnonblocking(PGconn *conn, int arg)
 int
 PQisnonblocking(const PGconn *conn)
 {
+	if (!conn || conn->status == CONNECTION_BAD)
+		return false;
 	return pqIsnonblocking(conn);
 }
 
@@ -3848,6 +3850,8 @@ PQisthreadsafe(void)
 int
 PQflush(PGconn *conn)
 {
+	if (!conn || conn->status == CONNECTION_BAD)
+		return -1;
 	return pqFlush(conn);
 }
 

src/interfaces/libpq/fe-secure-openssl.c

@@ -1806,7 +1806,7 @@ err:
 int
 PQdefaultSSLKeyPassHook_OpenSSL(char *buf, int size, PGconn *conn)
 {
-	if (conn->sslpassword)
+	if (conn && conn->sslpassword)
 	{
 		if (strlen(conn->sslpassword) + 1 > size)
 			fprintf(stderr, libpq_gettext("WARNING: sslpassword truncated\n"));