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.
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index c731ed1..5f320bd 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -407,6 +407,8 @@
     idx = i;
     return true;
   }
+  if (unlikely (in_error))
+    return false;
 
   assert (i <= out_len + (len - idx));