Fix not to assign null to strstr function

Author: ago-shi

contrib/win32/win32compat/w32-doexec.c

@@ -469,12 +469,15 @@ int do_exec_windows(struct ssh *ssh, Session *s, const char *command, int pty) {
 		}
 
 		//Passing the PRIVSEP_LOG_FD (STDERR_FILENO + 2) to sftp-server for logging
-		if(strstr(exec_command, "sftp-server.exe"))
-			if (posix_spawn_file_actions_adddup2(&actions, STDERR_FILENO + 2, SFTP_SERVER_LOG_FD) != 0) {
-				errno = EOTHER;
-				error("posix_spawn initialization failed");
-				goto cleanup;
+		if (command) {
+			if (strstr(exec_command, "sftp-server.exe")) {
+				if (posix_spawn_file_actions_adddup2(&actions, STDERR_FILENO + 2, SFTP_SERVER_LOG_FD) != 0) {
+					errno = EOTHER;
+					error("posix_spawn initialization failed");
+					goto cleanup;
+				}
 			}
+		}
 
 		if (posix_spawn(&pid, spawn_argv[0], &actions, NULL, spawn_argv, NULL) != 0) {
 			errno = EOTHER;