[sfnt] Avoid 'runtime error: applying zero offset to null pointer'.

* src/sfnt/ttsbit.c (tt_sbit_decoder_load_byte_aligned): Exit early if
`line` is NULL.

src/sfnt/ttsbit.c

@@ -730,6 +730,9 @@
     pitch      = bitmap->pitch;
     line       = bitmap->buffer;
 
+    if ( !line )
+      goto Exit;
+
     width  = decoder->metrics->width;
     height = decoder->metrics->height;