formdata: Check if length is too large for memory

- If the size of the length type (curl_off_t) is greater than the size
of the size_t type then check before allocating memory to make sure the
value of length will fit in a size_t without overflow. If it doesn't
then return CURLE_BAD_FUNCTION_ARGUMENT.

Bug: https://github.com/bagder/curl/issues/425#issuecomment-154518679
Reported-by: Steve Holme