Safer getcwd in DirAccess

Fix [-Wunused-result]

drivers/unix/dir_access_unix.cpp

@@ -309,7 +309,7 @@ Error DirAccessUnix::change_dir(String p_dir) {
 	// prev_dir is the directory we are changing out of
 	String prev_dir;
 	char real_current_dir_name[2048];
-	getcwd(real_current_dir_name, 2048);
+	ERR_FAIL_COND_V(getcwd(real_current_dir_name, 2048) == NULL, ERR_BUG);
 	if (prev_dir.parse_utf8(real_current_dir_name))
 		prev_dir = real_current_dir_name; //no utf8, maybe latin?
 
@@ -330,7 +330,7 @@ Error DirAccessUnix::change_dir(String p_dir) {
 
 	// the directory exists, so set current_dir to try_dir
 	current_dir = try_dir;
-	chdir(prev_dir.utf8().get_data());
+	ERR_FAIL_COND_V(chdir(prev_dir.utf8().get_data()) != 0, ERR_BUG);
 	return OK;
 }
 
@@ -405,7 +405,7 @@ DirAccessUnix::DirAccessUnix() {
 
 	// set current directory to an absolute path of the current directory
 	char real_current_dir_name[2048];
-	getcwd(real_current_dir_name, 2048);
+	ERR_FAIL_COND(getcwd(real_current_dir_name, 2048) == NULL);
 	if (current_dir.parse_utf8(real_current_dir_name))
 		current_dir = real_current_dir_name;