AP_Filesystem: give a ROMFS directory record back when opendir fails

opendir() took one of the four directory records, and kept it when the
directory turned out not to exist. Four such attempts left every later
ROMFS opendir() failing with ENFILE until a reboot; over MAVFTP, four
listings of a missing @ROMFS path were enough. It also returned without
setting errno.

Free the record and set ENOENT.

libraries/AP_Filesystem/AP_Filesystem_ROMFS.cpp

@@ -156,6 +156,7 @@ void *AP_Filesystem_ROMFS::opendir(const char *pathname)
     dir[idx].ofs = 0;
     dir[idx].path = strdup(pathname);
     if (!dir[idx].path) {
+        errno = ENOMEM;
         return nullptr;
     }
 
@@ -163,7 +164,10 @@ void *AP_Filesystem_ROMFS::opendir(const char *pathname)
     const char *name = AP_ROMFS::dir_list(dir[idx].path, dir[idx].ofs);
     dir[idx].ofs = 0;
     if (!name) {
-        // Directory does not exist
+        // Directory does not exist; give the record back
+        free(dir[idx].path);
+        dir[idx].path = nullptr;
+        errno = ENOENT;
         return nullptr;
     }