Remove some unneeded NULL checks

Found by Coverity; also see commit 85fb2af3.
Silence some Coverity warnings

None of this really matters.
af_scaletempo: use float division for rate

From what I understand the division is to align the dimension of the
value from seconds to milliseconds. Hard to tell whether the "rounding"
was intentional or not; I'm tipping on "not".

Found by Coverity.
ao_lavc: fix setting up AVFrame pointers

The caller set up the "start" pointer array using the number of planes,
the encode() function used the number of channels. This copied
uninitialized values for packed formats, which makes Coverity warn.
ao_oss: check whether setting samplerate succeeds

Independent from whether the samplerate was accepted or adjusted, errors
returned by the ioctl are fatal errors.

Found by Coverity.
ao_pcm: simplify

Also shuts up Coverity.
m_property: fix a typo

The wrong value was checked for an error. Oops.

Found by Coverity.
parse_configfile: check quote termination properly

This is some terrible code; I blame MPlayer legacy.

Found by Coverity.