Add asserts to *Guard pointer constructors...

... as we can't drop them. IMHO it's still better then just a
crash. Has to use NULL, as the API is pre-C++11 compatible.

Change-Id: Idb4395d14a9185f4d7d1c2f048ad950518eef133
Reviewed-on: https://gerrit.libreoffice.org/70465Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins

include/osl/mutex.hxx

@@ -22,6 +22,7 @@
 
 #include "osl/mutex.h"
 
+#include <cassert>
 
 namespace osl
 {
@@ -119,6 +120,7 @@ namespace osl
         */
         Guard(T * pT_) : pT(pT_)
         {
+            assert(pT != NULL);
             pT->acquire();
         }
 
@@ -153,6 +155,7 @@ namespace osl
         */
         ClearableGuard(T * pT_) : pT(pT_)
         {
+            assert(pT != NULL);
             pT->acquire();
         }