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.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>