Explicitly NUL terminate the buffer filled by fread().

The fix in r300649 was not sufficient to convince Coverity that the
buffer was NUL terminated, even with the buffer pre-zeroed. Swap
the size and nmemb arguments to fread() so that a valid lenght is
returned, which we can use to terminate the string in the buffer
at the correct location. This should also quiet the complaint about
the return value of fread() not being checked.

Reported by: Coverity
CID: 1019054, 1009614
MFC after: 1 week