When completing "help foo bar" if "foo" is not a real command, don't ask its NULL command object to complete the line.
When completing "help foo bar" if "foo" is not a real command, don't ask its NULL command object to complete the line.

git-svn-id: b33bab8abb5b18c12ee100cd7761ab452d00b2b0@143047 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb