vietnamese/vnconvert: Replace gets(3) with gets_s(3)

The long deprecated and unsafe gets(3) function has been removed since
FreeBSD 1300043.

Thus replace the occurences of that function in the code with gets_s(3)
which is a safer alternative.

• Bump PORTREVISION due to changes in the code/package.

PR: 238687
Reported by: emaste
MFH: 2019Q3