fatfs: Free path copy when f_open fails (FATFS_FSTAT)

When FATFS_FSTAT is enabled, open_r allocates a path copy for fstat/fchmod.
If f_open fails, close_r never runs, so the buffer was leaked. Free f->path
on the error path and clear the pointer, matching close_r.

See PR_DESCRIPTION.md for full description, implementation notes, and testing
suggestions.

Author: herbenderbler

PR_DESCRIPTION.md

@@ -0,0 +1,14 @@
+# FatFS: free path copy when `f_open` fails (`FATFS_FSTAT`)
+
+## Description
+
+When `FATFS_FSTAT` is enabled, `open_r` allocates a heap copy of the opened path for `fstat`/`fchmod`. If `f_open` failed, that buffer was never freed because `close_r` does not run on a failed open. Each failed open leaked `strlen(path) + 1` bytes until reboot. This change frees the path copy on the `f_open` error path and clears the pointer, matching cleanup in `close_r`.
+
+## Implementation Details
+
+- **`src/buddy/filesystem_fatfs.cpp`**: In `open_r`, inside `if (result != FR_OK)`, add a `FATFS_FSTAT`-guarded block that calls `free(f->path)` when non-null and sets `f->path` to `nullptr` before existing `FR_NO_FILESYSTEM` handling and `return -1`.
+
+## Test/Documentation
+
+- No automated test added; `FATFS_FSTAT` is optional (currently commented out in this file), so default CI builds do not exercise this block.
+- To exercise the change: define `FATFS_FSTAT`, build a target that uses the FatFS devoptab, and open a path that makes `f_open` fail (e.g. missing file on USB); confirm heap does not grow on repeated attempts (or use a host-side Memcheck model of the same control flow).

src/buddy/filesystem_fatfs.cpp

@@ -239,6 +239,12 @@ static int open_r(struct _reent *r, void *fileStruct, const char *path, int flag
     r->_errno = get_errno(result);
 
     if (result != FR_OK) {
+#ifdef FATFS_FSTAT
+        if (f->path) {
+            free(f->path);
+            f->path = nullptr;
+        }
+#endif
         if (result == FR_NO_FILESYSTEM) {
             // Displays exact message that file system on usb disk is not supported
             marlin_client::set_warning(WarningType::USBDriveUnsupportedFileSystem);