gst/smpte/gstsmpte.c: Some more sanity checks to make sure the input formats match and the input pads are actually ne...

Original commit message from CVS:
* gst/smpte/gstsmpte.c: (gst_smpte_collected):
Some more sanity checks to make sure the input formats match and the
input pads are actually negotiated, in case someone tries to feed
buffers from fakesrc or filesrc. Fixes #398299.
Also const-ify an array, just because we can.