Don't compute stateDB for non-existing block number

[aidan-kwon]

Proposed changes

1. In the changed logic, a break condition of the for loop can cause panic.
   The values of block should not be nil since block.NumberU64() is called in the following logic.

2. computeStateDB should find the state of the given block. However, it finds the state of the previous block of the given block.

The panic can be triggered when TraceBlock API is called with the block number 1. The API tries to lookup the previous state of the genesis.

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Further comments

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[KimKyungup]

LGTM.

[aidan-kwon]

@kjhman21 I am about to merge this PR. If you have any other opinions, please leave the comments.