formdata.c: Fixed compilation warning

formdata.c:390: warning: cast from pointer to integer of different size

Introduced in commit ca5f9341 this happens because a char*, which is
32-bits wide in 32-bit land, is being cast to a curl_off_t which is
64-bits wide where 64-bit integers are supported by the compiler.

This doesn't happen in 64-bit land as a pointer is the same size as a
curl_off_t.

This fix doesn't address the fact that a 64-bit value cannot be used
for CURLFORM_CONTENTLEN when set in a form array and compiled on a
32-bit platforms, it does at least suppress the compilation warning.