Avoid a memory leak if vfprintf(3) by always calling va_end(3); this

Avoid a memory leak if vfprintf(3) by always calling va_end(3); this
happens if vfprintf(3) runs out of memory.

Move the va_start(3) closer to the vfprintf(3) call; free the buffer
when if there is a failure condition and don't bother to realloc the
buffer before returning it.

Work done by Bruce Evans and Tim J. Robbins of the FreeBSD Project.