Bug 1525664 follow-up - Fix GDB pretty printer for JSString. r=jonco

I had to use try-except instead of "in" because if the value is a pointer type
(like JSString*) then "in" returned false.

I tested this with both 32-bit and 64-bit builds.

Differential Revision: https://phabricator.services.mozilla.com/D40978

--HG--
extra : moz-landing-system : lando

js/src/gdb/mozilla/JSString.py

@@ -43,10 +43,10 @@ class JSStringPtr(Common):
 
     def chars(self):
         d = self.value['d']
-        flags = d['flags_']
-        if 'length_' in d.type:
-            length = d['length_']
-        else:
+        flags = self.value['flags_']
+        try:
+            length = self.value['length_']
+        except gdb.error:
             # If we couldn't fetch the length directly, it must be stored
             # within `flags`.
             length = flags >> 32