doc/filters: fix @item syntax in showspectrum docs

In particular, fix warnings:
** Unknown command with braces `@item' (in doc/filters.texi l. 60XX)
doc/filters: apply various rendering changes to the showspectrum docs

In particular: always use @table @samp for showing constant tables, add a
few empty lines for improving readability, and specify default value when
missing.
ffv1enc: check for malloc failure

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffv1: check for malloc failure

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>