csplit: Fix check of fputs() return value, making csplit work again.

As of r295638, fputs() returns the number of bytes written (if not more than
INT_MAX). This broke csplit completely, since csplit assumed only success
only for the return value 0.

PR: 213510
Submitted by: J.R. Oldroyd
MFC after: 1 week
Relnotes: yes