eth: fix max gas price nil pointer error in replace transaction

[kyriediculous]

What does this pull request do? Explain your changes. (required)
Check that gpm.MaxGasPrice isn't nil in TransctionManager.replace()

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x7ff626d592eb]

goroutine 44 [running]:
math/big.(*Int).Cmp(0xc00107b5a0, 0x0, 0x1)
        D:/a/_temp/msys/msys64/mingw64/lib/go/src/math/big/int.go:328 +0x4b
github.com/livepeer/go-livepeer/eth.(*TransactionManager).replace(0xc0004340f0, 0xc001760750, 0x7ff6292630c0, 0x7ff628912a01, 0x7ff6292630c0)
        D:/a/go-livepeer/go-livepeer/eth/transactionManager.go:169 +0x1e8
github.com/livepeer/go-livepeer/eth.(*TransactionManager).checkTxLoop(0xc0004340f0)
        D:/a/go-livepeer/go-livepeer/eth/transactionManager.go:222 +0x2a9
github.com/livepeer/go-livepeer/eth.(*TransactionManager).Start(0xc0004340f0)
        D:/a/go-livepeer/go-livepeer/eth/transactionManager.go:124 +0x32
created by main.main
        D:/a/go-livepeer/go-livepeer/cmd/livepeer/livepeer.go:412 +0x2a9a

Specific updates (required)
added nil check

How did you test each of these updates (required)
Removed max gas price initial value from unit tests

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

LGTM