Priority big int conversion should not causing underflow

[yzang2019]

Describe your changes and provide context

This PR fixes the immunify bug report: https://bugs.immunefi.com/dashboard/submission/32602?resolvedFilter=unresolved

The actual problem is:
this section of code:

	// calculate the priority by dividing the total fee with the native gas limit (i.e. the effective native gas price)
	priority := fc.CalculatePriority(ctx, txData)
	ctx = ctx.WithPriority(priority.Int64())

priority is a big.Int that's converted to an int64 without first checking if such a conversion is possible with IsInt64(). If priority is negative, converting to int64 causes the final value to underflow and wrap around to a very large integer (for example MaxInt64 – max priority).

The code that sets priority for non-EVM transactions correctly calls IsInt64() (https://github.com/sei-protocol/sei-cosmos/blob/10546b70331d5f13e52b38acbf366a527566c3f1/x/auth/ante/validator_tx_fee.go#L77-L79), but the EVM version is missing this check.

Testing performed to validate your change

[philipsu522]

what if priority is not int64? should we set some default priority?

[yzang2019]

It will be default to 0 if not set

[yzang2019]

But maybe we can explicitly set it to 0 again

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 61.42%. Comparing base (aefdcf8) to head (0b6e9a5).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1787      +/-   ##
==========================================
- Coverage   61.47%   61.42%   -0.05%     
==========================================
  Files         257      257              
  Lines       22284    22289       +5     
==========================================
- Hits        13698    13690       -8     
- Misses       7629     7631       +2     
- Partials      957      968      +11     

Files	Coverage Δ
x/evm/ante/fee.go	59.75% <44.44%> (-4.72%)	⬇️

... and 7 files with indirect coverage changes