UserExpression::Evaluate only returns a non-empty ValueObjectSP
if it returns eExpressionCompleted.  Don't try to get the error
from the ValueObjectSP if that's not true.

I just have a report of this from the field, I don't know how
to make it fail yet.

<rdar://problem/29113004>

llvm-svn: 286170
diff --git a/lldb/source/Expression/UserExpression.cpp b/lldb/source/Expression/UserExpression.cpp
index f9cfaaa..52ef6fa 100644
--- a/lldb/source/Expression/UserExpression.cpp
+++ b/lldb/source/Expression/UserExpression.cpp
@@ -158,6 +158,7 @@
     if (log)
       log->Printf("== [UserExpression::Evaluate] Passed a NULL target, can't "
                   "run expressions.");
+    error.SetErrorString("expression passed a null target");
     return lldb::eExpressionSetupError;
   }