Simplify use of posix_assert in mutex.hpp

It is the job of the posix_assert macro to check
the value. No need to do it twice.

The patch also fixes some whitespace problems.

src/mutex.hpp

@@ -81,28 +81,24 @@ namespace zmq
         inline mutex_t ()
         {
             int rc = pthread_mutex_init (&mutex, NULL);
-            if (rc)
             posix_assert (rc);
         }
 
         inline ~mutex_t ()
         {
             int rc = pthread_mutex_destroy (&mutex);
-            if (rc)
             posix_assert (rc);
         }
 
         inline void lock ()
         {
             int rc = pthread_mutex_lock (&mutex);
-            if (rc)
             posix_assert (rc);
         }
 
         inline void unlock ()
         {
             int rc = pthread_mutex_unlock (&mutex);
-            if (rc)
             posix_assert (rc);
         }