Check tor_vasprintf for error return values.
In case of error, a negative value will be returned or NULL written into first supplied argument.
This patch uses both cases to comply with style in the specific files.
A tor_vasprintf error in process_vprintf would lead to a NULL dereference later on in buf_add, because the return value -1 casted to size_t would pass an assertion check inside of buf_add.
On the other hand, common systems will fail on such an operation, so it is not a huge difference to a simple assertion. Yet it is better to properly fail instead of relying on such behaviour on all systems.