[unsafe-to-break] Fix Use-of-uninitialized-value in unsafe_to_break_set_mask

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3011

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

@@ -372,7 +372,7 @@ preprocess_text_hangul (const hb_ot_shape_plan_t *plan,
 	    buffer->merge_out_clusters (start, end);
 	  continue;
 	}
-	else if (!tindex)
+	else if ((!tindex && buffer->idx + 1 < count && isT (buffer->cur(+1).codepoint)))
 	  buffer->unsafe_to_break (buffer->idx, buffer->idx + 2); /* Mark unsafe between LV and T. */
       }