Make MoveFileEx fail with ERROR_INVALID_PARAMETER (better than
crashing) when the source file operand is NULL.

diff --git a/files/file.c b/files/file.c
index 9c8d115..0993492 100644
--- a/files/file.c
+++ b/files/file.c
@@ -2006,6 +2006,11 @@
 
     TRACE("(%s,%s,%04lx)\n", fn1, fn2, flag);
 
+    if (!fn1) {
+        SetLastError(ERROR_INVALID_PARAMETER);
+        return FALSE;
+    }
+
     if (!DOSFS_GetFullName( fn1, TRUE, &full_name1 )) return FALSE;
 
     if (fn2)  /* !fn2 means delete fn1 */