Add return for `AllocBlock` and `Scrap_AllocBlock`

In the event a block for a texture cannot be allocated (or scraped)
enforce a return of `0` by the function rather than it to fall off the
end.

If a block cannot be allocated the function actually calls `Sys_Error`,
and terminates as a result. However, if this functionality is modified
it leaves the result of a call to `AllocBlock`, or `Scrap_AllocBlock`
__undefined__.

The return value of `0` was taken from the same functions in the Quake
World code base (the statement `return 0` is the only line in which
these functions differ).