made gdb pretty-printing script more robust when printing uninitializ…

…ed vec.

I based this solution on my reading of:

https://rethinkdb.com/blog/make-debugging-easier-with-custom-pretty-printers#what-is-still-to-be-done

That post claims that there is no clean way to check for garbage pointers, and
so this PR adopts the same solution of tentatively attempting to convert a
dererence to a string, which throws a clean exception on garbage that we can
catch and recover from.

I only made the change to vec and not the other pretty printers because I wanted
to focus my effort on the simplest thing that would resolve issue #64343. In
particular, I *considered* generalizing this fix to work on the other datatypes
in the pretty-printing support library, but I don't want to invest effort in
that until after we resolve our overall debugging support strategy; see also
issues #60826 and #65564.

src/etc/gdb_rust_pretty_printing.py

@@ -284,10 +284,20 @@ def to_string(self):
                 ("(len: %i, cap: %i)" % (length, cap)))
 
     def children(self):
+        saw_inaccessible = False
         (length, data_ptr, cap) = rustpp.extract_length_ptr_and_cap_from_std_vec(self.__val)
         gdb_ptr = data_ptr.get_wrapped_value()
         for index in xrange(0, length):
-            yield (str(index), (gdb_ptr + index).dereference())
+            if saw_inaccessible:
+                return
+            try:
+                # rust-lang/rust#64343: passing deref expr to `str` allows
+                # catching exception on garbage pointer
+                str((gdb_ptr + index).dereference())
+                yield (str(index), (gdb_ptr + index).dereference())
+            except RuntimeError:
+                saw_inaccessible = True
+                yield (str(index), "inaccessible")
 
 
 class RustStdVecDequePrinter(object):