DecCoin bugs

[jaekwon]

MulDec can cause a product to be zero due to rounding. MulDecTruncate, QuoDec, and QuoDecTruncated as well. ParseDecCoin can return a zero coin but ParseDecCoins should not, so ParseDecCoins also has a bug

[rigelrozanski]

Oh I understand - various dec operations returning 0 due to rounding is expected behaviour, but the presence of 0 denoms within DecCoins is a bug, thus we need to check for introduction of 0 coins after dec operations on deccoins... correct?

[alexanderbez]

ParseDecCoins: I don't see how this is broken. It calls IsValid on the final result and returns an error if invalid. It's the exact same logic as ParseCoins. Is ParseCoins also broken?

The rest, I'll address: QuoDecTruncate, QuoDec, MulDecTruncate

[jaekwon]

parsedeccoins, i dunno about parsecoins, maybe, but parsedeccoins should remove zero any deccoins before returning.

a zero deccoin is valid but not in a deccoins.

[alexanderbez]

parsedeccoins, i dunno about parsecoins, maybe, but parsedeccoins should remove zero any deccoins before returning.

a zero deccoin is valid but not in a deccoins.

Ref: #4095