libbsnmptools: avoid uninitialized snmptoolctx->passwd with empty password

The removed check left snmptoolctx->passwd pointer to uninitialized
memory. Always calling strlcpy(3) would guarantee that with empty
password it will point to empty string.

Submitted by: markj
PR: 283909

(cherry picked from commit 3999a860d6e899de98b1025317d2d0ef1f83255f)
(cherry picked from commit 0bfbd30663b68a851ebf24667d121c6891c86827)