Address a corner-case associated with the call-function-once optimization

of check-in [663f5dd32d9db832] that was causing a (harmless) use of an
uninitialized bytecode register.
dbsqlfuzz 8a44f675401a8b1f68a43bf813c4f4f72ad8f0ea

FossilOrigin-Name: b37ac2d020873b78efa164036db4056406a67330679f850775da520cd8027cb9