[GC] Pin Key PMCs During Method Lookup

[chromatic]

In src/objects.c, around line 82, the function find_vtable_meth_ns() creates a 
Key PMC to perform a lookup:

        PMC   * const key = VTABLE_nextkey_keyed(interp, key_new(interp), ns,
                ITERATE_FROM_START);

If a GC run happens in the rest of this function, it could collect the newly 
created Key -- PMCs created and held outside of structures visible to GC are 
vulnerable to collection.

Unfortunately, I can't reproduce the problem on Linux on x86.  However, I do 
think that storing the key temporarily in a place where it will get marked 
appropriately may fix things.  Note that unpinning the key is important at 
all exit points.

I hope this patch improves things for Windows users -- or at least stops them 
from getting worse.

-- c


=== src/objects.c
==================================================================
--- src/objects.c	(revision 3990)
+++ src/objects.c	(local)
@@ -90,6 +90,8 @@
 
     int         j;
 
+    dod_register_pmc(interp, key);
+
     for (j = 0; j < k; ++j) {
         STRING * const ns_key = (STRING *)parrot_hash_get_idx(interp,
                             (Hash *)PMC_struct_val(ns), key);
@@ -99,9 +101,13 @@
         if (res->vtable->base_type == enum_class_Sub &&
                (PMC_sub(res)->vtable_index == vtable_index ||
                 string_compare(interp, meth_str, ns_key) == 0))
+        {
+            dod_unregister_pmc(interp, key);
             return res;
+        }
     }
 
+    dod_unregister_pmc(interp, key);
     return PMCNULL;
 }