patch 8.2.1975: Win32: memory leak when encoding conversion fails

Problem:    Win32: memory leak when encoding conversion fails.
Solution:   Free the allocated memory. (Ken Takata, closes #7277)
diff --git a/src/os_win32.c b/src/os_win32.c
index 52573fe..60c0463 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -3532,7 +3532,10 @@
 
 	wn = enc_to_utf16(fname, NULL);
 	if (wn == NULL)
+	{
+	    vim_free(p);
 	    return NULL;
+	}
 
 	// Try to retrieve the entire security descriptor.
 	err = GetNamedSecurityInfoW(