After failing to read a non-existing file, the shell tried to execute the resulting empty program, which resulted in a null pointer dereference. This patch avoids execution of empty programs.

Review URL: http://codereview.chromium.org/6777005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7473 ce2b1a6d-e550-0410-aec6-3dcde31c8c00