Try to better handle OOM situations

Fixes assert fail in https://github.com/behdad/harfbuzz/issues/161
with libharfbuzz-fuzzing.

src/hb-ot-shape-complex-use.cc

diff --git a/src/hb-ot-shape-complex-use.cc b/src/hb-ot-shape-complex-use.cc
index cc01fd3..dc2d7af 100644
--- a/src/hb-ot-shape-complex-use.cc
+++ b/src/hb-ot-shape-complex-use.cc
@@ -490,11 +490,6 @@
   if (likely (!has_broken_syllables))
     return;
 
-
-  hb_codepoint_t dottedcircle_glyph;
-  if (!font->get_glyph (0x25CCu, 0, &dottedcircle_glyph))
-    return;
-
   hb_glyph_info_t dottedcircle = {0};
   if (!font->get_glyph (0x25CCu, 0, &dottedcircle.codepoint))
     return;
@@ -503,9 +498,8 @@
   buffer->clear_output ();
 
   buffer->idx = 0;
-
   unsigned int last_syllable = 0;
-  while (buffer->idx < buffer->len)
+  while (buffer->idx < buffer->len && !buffer->in_error)
   {
     unsigned int syllable = buffer->cur().syllable();
     syllable_type_t syllable_type = (syllable_type_t) (syllable & 0x0F);