gdb: initialize interp::next

This field is never initialized, it seems to me like it would be a good
idea to initialize it to nullptr to avoid bad surprises.

Change-Id: I8c04319d564f5d385d8bf0acee758f6ce28b4447
Reviewed-By: Tom Tromey <tom@tromey.com>

gdb/interps.h

@@ -87,7 +87,7 @@ private:
 public:
   /* Interpreters are stored in a linked list, this is the next
      one...  */
-  struct interp *next;
+  interp *next = nullptr;
 
   /* Has the init method been run?  */
   bool inited = false;