gst/law/: The encoder can't really renegotiate at the time they perform a pad-alloc so make the srcpads use fixed caps.

Original commit message from CVS:
* gst/law/alaw-encode.c: (gst_alaw_enc_init), (gst_alaw_enc_chain):
* gst/law/mulaw-conversion.c:
* gst/law/mulaw-encode.c: (gst_mulawenc_init),
(gst_mulawenc_chain):
The encoder can't really renegotiate at the time they perform a
pad-alloc so make the srcpads use fixed caps.
Check the buffer size after a pad-alloc because the returned size might
not be right when the downstream element does not know the size of the
new buffer (capsfilter). Fixes #549073.