chaingen: Allow 32-bit compile.
chaingen: Allow 32-bit compile.

The go compiler treats the math.MaxUint32 constant as an integer in the
error print which causes a compile error.  This casts the constant to
avoid that thus allows the code to compile on 32-bit architectures.

jrick/dcrd,jrick/dcrd,jcmincke/dcrd,jrick/btcd,decred/dcrd,jcmincke/dcrd,jrick/btcd,jrick/btcd,decred/dcrd,jcmincke/dcrd