[EditLine] Check string pointers before dereferencing them.
[EditLine] Check string pointers before dereferencing them.

Get*AtIndex() can return nullptr. This only happens in the swift
REPL support, so it's hard to test upstream.

<rdar://problem/50875178>

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@361078 91177308-0d34-0410-b5e6-96231b3b80d8

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