[GPOS] Fix assert fail introduced recently

Was introduced in 8708b9e081.

If these lookups are recursed to from (Chain)Context out-of-order,
it was possible that last_base > buffer->idx, in which case we
were attaching marks to a base after them... and an assertion
was failing fortunately.

Fixes https://oss-fuzz.com/testcase-detail/6377756666757120

src/OT/Layout/GPOS/MarkBasePosFormat1.hh

@@ -122,6 +122,11 @@ struct MarkBasePosFormat1_2
     hb_ot_apply_context_t::skipping_iterator_t &skippy_iter = c->iter_input;
     skippy_iter.set_lookup_props (LookupFlag::IgnoreMarks);
 
+    if (c->last_base_until > buffer->idx)
+    {
+      c->last_base_until = 0;
+      c->last_base = -1;
+    }
     unsigned j;
     for (j = buffer->idx; j > c->last_base_until; j--)
     {

src/OT/Layout/GPOS/MarkLigPosFormat1.hh

@@ -104,6 +104,11 @@ struct MarkLigPosFormat1_2
     hb_ot_apply_context_t::skipping_iterator_t &skippy_iter = c->iter_input;
     skippy_iter.set_lookup_props (LookupFlag::IgnoreMarks);
 
+    if (c->last_base_until > buffer->idx)
+    {
+      c->last_base_until = 0;
+      c->last_base = -1;
+    }
     unsigned j;
     for (j = buffer->idx; j > c->last_base_until; j--)
     {