[subset] Fail subset if a table fails to sanitize.

src/hb-subset.cc

diff --git a/src/hb-subset.cc b/src/hb-subset.cc
index a9f599f..b7d3e37 100644
--- a/src/hb-subset.cc
+++ b/src/hb-subset.cc
@@ -83,16 +83,19 @@
 static bool
 _subset (hb_subset_plan_t *plan)
 {
-    OT::Sanitizer<TableType> sanitizer;
-    hb_blob_t *source_blob = sanitizer.sanitize (plan->source->reference_table (TableType::tableTag));
-    const TableType *table = OT::Sanitizer<TableType>::lock_instance (source_blob);
-    hb_bool_t result = table->subset(plan);
+  OT::Sanitizer<TableType> sanitizer;
 
-    hb_blob_destroy (source_blob);
+  hb_blob_t *source_blob = sanitizer.sanitize (plan->source->reference_table (TableType::tableTag));
+  const TableType *table = OT::Sanitizer<TableType>::lock_instance (source_blob);
 
-    hb_tag_t tag = TableType::tableTag;
-    DEBUG_MSG(SUBSET, nullptr, "OT::%c%c%c%c::subset %s", HB_UNTAG(tag), result ? "success" : "FAILED!");
-    return result;
+  hb_bool_t result = false;
+  if (table != &OT::Null(TableType))
+    result = table->subset(plan);
+
+  hb_blob_destroy (source_blob);
+  hb_tag_t tag = TableType::tableTag;
+  DEBUG_MSG(SUBSET, nullptr, "OT::%c%c%c%c::subset %s", HB_UNTAG(tag), result ? "success" : "FAILED!");
+  return result;
 }