wt-status: fix possible use of uninitialized variable

In wt_status_print_change_data, we accept a change_type flag that is meant to be either WT_STATUS_UPDATED or WT_STATUS_CHANGED. We then switch() on this value to set the local variable "status" for each case, but do not provide a fallback "default" label to the switch statement. As a result, the compiler realizes that "status" might be unset, and complains with a warning. To silence this warning, we use the "int status = status" trick. This is correct with the current code, as all callers provide one of the two expected change_type flags. However, it's also a maintenance trap, as there is nothing to prevent future callers from passing another flag, nor to document this assumption. Instead of using the "x = x" hack, let's handle the default case in the switch() statement with a die("BUG"). That tells the compiler and any readers of the code exactly what the function's input assumptions are. We could also convert the flag to an enum, which would provide a compile-time check on the function input. However, since these flags are part of a larger enum, that would make the code unnecessarily complex (we would have to make a new enum with just the two flags, and then convert it to the old enum for passing to sub-functions). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>