Ptr byte to cstring?

[jrfondren]

C strings are zero-terminated, and FreeImage_AcquireMemory isn't filling the 
buffer with non-zero bytes for `len()` to count. Even if it did, the assertion 
would fail as a C string can't have a length equal to the memory backing it 
without `len()` reading out of bounds.

Consider:
    
    
    var buffer = [byte('a'), byte('b'), 0, 0, 0]
    let cstr = cast[cstring](buffer[0].addr)
    doAssert cstr == "ab"
    doAssert cstr.len == 2
    buffer[2] = byte('c')
    doAssert cstr.len == 3
    
    
    Run