od: reinstate half float validation check

od: reinstate half float validation check

Reinstate check removed in commit 56aa549a0 so that we
disallow -f2 when configured with utils_cv_ieee_16_bit_supported=no.
Otherwise the output routines will consume floats,
i.e. 4 bytes at a time.  Without this extra check
the tests/od/od-endian.sh will fail with this configuration.

* src/od.c (decode_one_format): Reinstate the explicit check
for this configuration edge case.