apimutex.c: Add _Assert for NULL pointer access

CodeSonar detects a possible NULL deference here. But it should never
occur in tested code. Memory for the API Mutexes is reserved by confdefs.h
and are all preallocated when the class of objects is initialized.
Allocating a single instance should never fail.

cpukit/score/src/apimutex.c

@@ -61,6 +61,8 @@ void _API_Mutex_Allocate(
   mutex = (API_Mutex_Control *)
     _Objects_Allocate_unprotected( &_API_Mutex_Information );
 
+  _Assert( mutex != NULL );
+
   _CORE_mutex_Initialize( &mutex->Mutex, NULL, &attr, false );
 
   _Objects_Open_u32( &_API_Mutex_Information, &mutex->Object, 1 );