gsupplicant: Reassign best_bss pointer when that bss is removed

When the bss to which best_bss points to is removed, the pointer also
needs to be reassigned, or reference to already deallocated memory may
happen later on.

[connman] gsupplicant: Reassign best_bss pointer when that bss is removed
Merge pull request #107 from hmallat/best-bss-reassignment

gsupplicant: Reassign best_bss pointer when that bss is removed