FFI: Ensure library is loaded before de-serializing FFI types.

Reported by ImagicTheCat.

doc/ext_buffer.html

@@ -448,6 +448,10 @@ encoded data. The stand-alone function throws when there's left-over
 data after decoding a single top-level object. The buffer method leaves
 any left-over data in the buffer.
 </p>
+<p>
+Attempting to de-serialize an FFI type will throw an error, if the FFI
+library is not built-in or has not been loaded, yet.
+</p>
 
 <h3 id="serialize_options">Serialization Options</h3>
 <p>

src/lj_serialize.c

@@ -417,6 +417,7 @@ static char *serialize_get(char *r, SBufExt *sbx, TValue *o)
     uint32_t sz = tp == SER_TAG_COMPLEX ? 16 : 8;
     GCcdata *cd;
     if (LJ_UNLIKELY(r + sz > w)) goto eob;
+    if (LJ_UNLIKELY(!ctype_ctsG(G(sbufL(sbx))))) goto badtag;
     cd = lj_cdata_new_(sbufL(sbx),
 	   tp == SER_TAG_INT64 ? CTID_INT64 :
 	   tp == SER_TAG_UINT64 ? CTID_UINT64 : CTID_COMPLEX_DOUBLE,