Fix right shift in GetBlockValue() when nHeight is greater than 3870720

The C++ right shift operation is undefined when the right operand is greater than
or equal to the length in bits of the promoted left operand.
On Intel x86 architecture, a right shift is translated into a "sar" or
"shrd" instruction. When (nHeight / 60480) is greater than or equal to 64,
it is observed that these right shift instructions will shift nSubsidy by
((nHeight / 60480) mod 64). For example, at block 3870720 (=60480*64),
the original GetBlockValue() returns 2048*COINS instead of 1*COINS.
This patch fixed this issue and GetBlockValue() will return 1*COINS for
any block height greater than 665279.