Add asserts to prevent mysterious hangs in a non-thread-safe gmock build.

include/gtest/internal/gtest-linked_ptr.h

@@ -110,7 +110,12 @@ class linked_ptr_internal {
     MutexLock lock(&g_linked_ptr_mutex);
 
     linked_ptr_internal const* p = ptr;
-    while (p->next_ != ptr) p = p->next_;
+    while (p->next_ != ptr) {
+      assert(p->next_ != this &&
+             "Trying to join() a linked ring we are already in. "
+             "Is GMock thread safety enabled?");
+      p = p->next_;
+    }
     p->next_ = this;
     next_ = ptr;
   }
@@ -123,7 +128,12 @@ class linked_ptr_internal {
 
     if (next_ == this) return true;
     linked_ptr_internal const* p = next_;
-    while (p->next_ != this) p = p->next_;
+    while (p->next_ != this) {
+      assert(p->next_ != next_ &&
+             "Trying to depart() a linked ring we are not in. "
+             "Is GMock thread safety enabled?");
+      p = p->next_;
+    }
     p->next_ = next_;
     return false;
   }