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
In general, I'm against adding code in the library to check for situations where the behavior is undefined.  There are three arguments to pmemlog_appendv(), all of which could be invalid in various ways.  plp could be wrong, iov could be wrong, iovcnt could be wrong.  Seems like the single most likely error would be that iovcnt is too large, forcing us to run off the end of the iov array.  iovcnt < 0 seems like a very random check of one unlikely condition.  It adds code to a path that is meant to be very low latency and provides little benefit.