ntdll: Avoid closing a handle twice.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/ntdll/loader.c

@@ -2282,6 +2282,7 @@ static NTSTATUS open_dll_file( UNICODE_STRING *nt_name, WINE_MODREF **pwm, HANDL
             TRACE( "%s is for arch %x, continuing search\n", debugstr_us(nt_name), image_info->Machine );
             status = STATUS_IMAGE_MACHINE_TYPE_MISMATCH;
             NtClose( *mapping );
+            *mapping = NULL;
         }
     }
     NtClose( handle );