Fix incorrect calculation in shm_mq_receive.

 Fix incorrect calculation in shm_mq_receive.
 
 If some, but not all, of the length word has already been read, and the
 next attempt to read sees exactly the number of bytes needed to complete
 the length word, or fewer, then we'll incorrectly read less than all of
 the available data.
 
 Antonin Houska