Change CommandObjectTargetSymbolsAdd to require that a target exists;
fixes crash of the form

% lldb
(lldb) target symbols add /tmp/symbols.dSYM
(lldb) Killed: 9

<rdar://problem/13139481> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@174267 91177308-0d34-0410-b5e6-96231b3b80d8