libpq: Add pqReleaseConnHosts function

In a follow up commit we'll need to free this connhost field in a
function defined in fe-cancel.c, so here we extract the logic to a
dedicated extern function.

Author: Jelte Fennema-Nio <jelte.fennema@microsoft.com>
Discussion: https://postgr.es/m/AM5PR83MB0178D3B31CA1B6EC4A8ECC42F7529@AM5PR83MB0178.EURPRD83.prod.outlook.com

src/interfaces/libpq/fe-connect.c

@@ -4349,19 +4349,7 @@ freePGconn(PGconn *conn)
 		free(conn->events[i].name);
 	}
 
-	/* clean up pg_conn_host structures */
-	for (int i = 0; i < conn->nconnhost; ++i)
-	{
-		free(conn->connhost[i].host);
-		free(conn->connhost[i].hostaddr);
-		free(conn->connhost[i].port);
-		if (conn->connhost[i].password != NULL)
-		{
-			explicit_bzero(conn->connhost[i].password, strlen(conn->connhost[i].password));
-			free(conn->connhost[i].password);
-		}
-	}
-	free(conn->connhost);
+	pqReleaseConnHosts(conn);
 
 	free(conn->client_encoding_initial);
 	free(conn->events);
@@ -4423,6 +4411,31 @@ freePGconn(PGconn *conn)
 	free(conn);
 }
 
+/*
+ * pqReleaseConnHosts
+ *	 - Free the host list in the PGconn.
+ */
+void
+pqReleaseConnHosts(PGconn *conn)
+{
+	if (conn->connhost)
+	{
+		for (int i = 0; i < conn->nconnhost; ++i)
+		{
+			free(conn->connhost[i].host);
+			free(conn->connhost[i].hostaddr);
+			free(conn->connhost[i].port);
+			if (conn->connhost[i].password != NULL)
+			{
+				explicit_bzero(conn->connhost[i].password,
+							   strlen(conn->connhost[i].password));
+				free(conn->connhost[i].password);
+			}
+		}
+		free(conn->connhost);
+	}
+}
+
 /*
  * store_conn_addrinfo
  *	 - copy addrinfo to PGconn object

src/interfaces/libpq/libpq-int.h

@@ -678,6 +678,7 @@ extern void pqDropConnection(PGconn *conn, bool flushInput);
 #if defined(WIN32) && defined(SIO_KEEPALIVE_VALS)
 extern int	pqSetKeepalivesWin32(pgsocket sock, int idle, int interval);
 #endif
+extern void pqReleaseConnHosts(PGconn *conn);
 extern int	pqPacketSend(PGconn *conn, char pack_type,
 						 const void *buf, size_t buf_len);
 extern bool pqGetHomeDirectory(char *buf, int bufsize);