Prevent UB on signed overflow.

Prevent UB on signed overflow.

Overflowing a signed integer in C is an undefined behaviour.
It is possible to trigger this undefined behaviour in tor_asprintf on
Windows or systems lacking vasprintf.

On these systems, eiter _vscprintf or vsnprintf is called to retrieve
the required amount of bytes to hold the string. These functions can
return INT_MAX. The easiest way to recreate this is the use of a
specially crafted configuration file, e.g. containing the line:

FirewallPorts AAAAA<in total 2147483610 As>

This line triggers the needed tor_asprintf call which eventually
leads to an INT_MAX return value from _vscprintf or vsnprintf.

The needed byte for \0 is added to the result, triggering the
overflow and therefore the undefined behaviour.

Casting the value to size_t before addition fixes the behaviour.

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