[lldb-mi] Fix assignment operator in CMIUtilString

Fix assignment operator in CMIUtilString to not crash the debugger if it
is passed a NULL pointer, which can happen in code like the following:

    SBValue val;
    CMIUtilString s;
    //...
    s = val.GetSummary();

Patch from evgeny.leviant@gmail.com
Reviewed by: clayborg, ki.stfu
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D13094

llvm-svn: 248597