Vector length might not be a multiple of 4 if, for example, it's a
vector of uint8_t or uint16_t. However, the actual memory allocated to
the vector should be 4 byte aligned, so test for that instead.
Added command "clearvec" to plugin "vectors". Takes a list of addresses
of vectors (or pointers to vectors), and sets each vector to zero length
by setting it's end to it's start. Leaks memory, of course, but this
is just to poke at the game to see what happens when a vector's
contents disappear.