summary: another band-aid for missing stacktrace

The following error was reported:

	UndefVarError: `bt` not defined

Previously, a different error was reported where bt was a 0-length
vector. Why the stacktrace is missing is not known in either case.

src/exception_summary.jl

@@ -143,9 +143,10 @@ function _summarize_exception(io::IO, exc, stack; prefix = nothing)
         end
     end
     # Now print just the very first frame we've collected:
-    if isempty(bt)
+    if ! @isdefined :bt || isempty(bt)
         # A report was received about bt being a 0-element Vector. It's not clear why the
         # stacktrace is missing, but this should tide us over in the meantime.
+        # Then another report was received about bt not being defined.
         _indent_println(io, "no stacktrace available")
     else
         (frame, n) = bt[1]