Fix error message when opening a mailbox with no read permission. (closes #3934)

Fix error message when opening a mailbox with no read permission. (closes #3934)

ctx->mx_ops ends up being NULL for both the case that ctx->magic is 0
and -1.  This meant the mutt_perror() error message was never being
printed, because the check for ctx->mx_ops == NULL was taking place
first.

Move the "ctx->magic == -1" check first, so mutt will print out an
appropriate perror message in that case.