Fix -V to fail invalid input safely

Fix -V to fail invalid input safely

This change fixes CVE-2018-14879.

get_next_file() did not check the return value of strlen() and
underflowed an array index if the line read by fgets() from the file
started with \0. This caused an out-of-bounds read and could cause a
write. Add the missing check.

This vulnerability was discovered by Brian Carpenter & Geeknik Labs.

Cherry picked from 9ba91381954ad325ea4fd26b9c65a8bd9a2a85b6
in 4.9 branch.