[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

lldb/tools/lldb-mi/MIUtilString.cpp

@@ -63,14 +63,7 @@ CMIUtilString::CMIUtilString(const std::string& vrStr)
 //--
 CMIUtilString &CMIUtilString::operator=(const char *vpRhs)
 {
-    if (*this == vpRhs)
-        return *this;
-
-    if (vpRhs != nullptr)
-    {
-        assign(vpRhs);
-    }
-
+    assign(vpRhs);
     return *this;
 }