Fix nullptr deref in archive

Hardened toolchains (eg, mainline Fedora) no longer tolerate null
derefs that were silently trapped/ignored in earlier versions.  This
eliminates a nullptr deref in archive.hpp that was failing several
unit tests (and causing crashes) when trying to manipulate
default-constructed archive objects.

libsrc/core/archive.hpp

@@ -225,7 +225,8 @@ namespace ngcore
       (*this) & size;
       if(Input())
         v.resize(size);
-      Do(&v[0], size);
+      if(size > 0) // can't deref v[0] if it doesn't exist
+        Do(&v[0], size);
       return (*this);
     }