[Backtracing][Linux] Handle inaccessible memory properly in Docker.

Thanks to a missing `-` sign, we were returning garbage rather than
indicating that the memory region in question was inaccessible.  This
mainly affects register dumps (since that's the time we expect to have
to cope with out-of-bounds reads).

rdar://117900760

Author: al45tair

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -668,7 +668,7 @@ memserver_read(void *to, const void *from, size_t len) {
       memcpy(to, from, len);
       return len;
     } else {
-      return 1;
+      return -1;
     }
   }
 }