[API CHANGE] Restore old exteral API behaviour

Commit d38f9934 ("[API CHANGE]
Asserts to never clear already cleared guard") changes the API
in a way, which can silently break compiling code for no reason.

As discussed on the mailing list, this uses LIBO_INTERNAL_ONLY to
restores the old behaviour for external API users.

Change-Id: I43c9b955c1c8f5402588fcea2bfc38ab6fa9f263
Reviewed-on: https://gerrit.libreoffice.org/70528Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins

include/osl/mutex.hxx

@@ -185,9 +185,15 @@ namespace osl
         */
         void clear()
         {
+#ifdef LIBO_INTERNAL_ONLY
             assert(pT);
-            pT->release();
-            pT = NULL;
+#else
+            if (pT)
+#endif
+            {
+                pT->release();
+                pT = NULL;
+            }
         }
     };
 
@@ -226,9 +232,14 @@ namespace osl
         */
         void reset()
         {
+#ifdef LIBO_INTERNAL_ONLY
             assert(!this->pT);
-            this->pT = pResetT;
-            this->pT->acquire();
+#endif
+            if (pResetT)
+            {
+                this->pT = pResetT;
+                this->pT->acquire();
+            }
         }
     };