core: Assert on memchunk divisibility by sample spec in pa_memblockq_push().

Earlier, -1 was returned if the memchunk size was not a multiple of the frame
size. Now, it is verified unconditionally through an assertion. Error code -1
is still returned when the memblock queue is full.

In those few cases where the return value of pa_memblockq_push() is checked,
an overflow is assumed to be the reason in case an error code is returned.

src/pulsecore/memblockq.h

@@ -51,7 +51,7 @@ typedef struct pa_memblockq pa_memblockq;
 
    - ss:        Sample spec describing the queue contents. Only multiples
                 of the frame size as implied by the sample spec are
-                popped from the queue or should be pushed into it.
+                allowed into the queue or can be popped from it.
 
    - prebuf:    If the queue runs empty wait until this many bytes are in
                 queue again before passing the first byte out. If set