log: fix pmemlog_appendv() if iovcnt is less than 0

pmemlog_appendv() should return -1 and set errno to EINVAL
when vector count iovcnt is less than zero.

Ref: pmem/issues#19
Yeah, it makes sense.
However, in this particular case, it's just one additional if(), which probably has no impact on the performance/latency.

The idea of the fix comes from the fact that the `pmemlog_appendv` may be expected to work *"in a manner similar to `writev(2)`,"* so it should probably perform the same checks on input arguments as the `writev()` does.

Personally, I don't see a problem with adding this check, but I'm also fine with leaving the code as is and adding a note in the man pages.