Avoid buffer->move_to() in case of buffer error

Fixes https://github.com/behdad/harfbuzz/issues/223

Right now we cannot test this because it has to be tested using hb-fuzzer.
We should move all fuzzing tests from test/shaping/tests/fuzzed.tests to
test/fuzzing/ and have its own test runner.  At that point, should add
test from this issue as well.

src/hb-buffer.cc

@@ -407,6 +407,8 @@ hb_buffer_t::move_to (unsigned int i)
     idx = i;
     return true;
   }
+  if (unlikely (in_error))
+    return false;
 
   assert (i <= out_len + (len - idx));
 

src/hb-ot-layout-gsubgpos-private.hh

@@ -971,7 +971,7 @@ static inline bool apply_lookup (hb_apply_context_t *c,
       match_positions[j] += delta;
   }
 
-  for (unsigned int i = 0; i < lookupCount; i++)
+  for (unsigned int i = 0; i < lookupCount && !buffer->in_error; i++)
   {
     unsigned int idx = lookupRecord[i].sequenceIndex;
     if (idx >= count)