extokens: fix missing NUL terminator append in GVPR tokenization

extokens: fix missing NUL terminator append in GVPR tokenization

This is the second half of a bug fix following the prior commit.

Commit 971293551421455a0d939b9f8cea17356b7968f8 refactored this code to avoid
the use of an SFIO buffer, inadvertently introducing a bug. The change did not
account for the source buffer not being NUL terminated. This fix wicks closer to
the original code, not assuming a NUL terminator and copying a known number of
bytes into the destination.

Fixes #2138.