It's safer to check the buffer size is large enough to hold the error
message before creating the slice from raw parts.

Also, this patch remove the need for `last_error`, simplifying the
code a little bit. The `length` variable is casted to `usize` once.

Returning the error message's length + 1 mimics the
`wasmer_last_error_length` function that counts the trailing null
byte.

…sage`.