mesh: net_key_status only pull one key idx
mesh: net_key_status only pull one key idx

Fixes bug where the config client's net_key_status handler would attempt
to pull two key indexes from a message which only holds one.

this is port of 2b33cf399cd21fc7e3ddabc06fcb137d0b9d5074

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble