Ganache provider crashes when evm_revert is passed an invalid snapshotId

[davidmurdoch]

If an object is passed to evm_revert the ganache provider will crash with

Error: invalid type
      at Object.exports.toBuffer (node_modules/ethereumjs-util/dist/index.js:183:13)
      at Object.exports.bufferToInt (node_modules/ethereumjs-util/dist/index.js:196:25)
      at StateManager.revert (lib/statemanager.js:813:22)
      at GethApiDouble.evm_revert (lib/subproviders/geth_api_double.js:521:14)
      at GethApiDouble.handleRequest (lib/subproviders/geth_api_double.js:106:10)
      at next (node_modules/web3-provider-engine/index.js:116:18)
      at GethDefaults.handleRequest (lib/subproviders/gethdefaults.js:15:12)
      at next (node_modules/web3-provider-engine/index.js:116:18)
      at SubscriptionSubprovider.FilterSubprovider.handleRequest (node_modules/web3-provider-engine/subproviders/filters.js:89:7)
      at SubscriptionSubprovider.handleRequest (node_modules/web3-provider-engine/subproviders/subscriptions.js:136:49)
      at next (node_modules/web3-provider-engine/index.js:116:18)
      at DelayedBlockFilter.handleRequest (lib/subproviders/delayedblockfilter.js:31:3)
      at next (node_modules/web3-provider-engine/index.js:116:18)
      at RequestFunnel.handleRequest (lib/subproviders/requestfunnel.js:32:12)
      at next (node_modules/web3-provider-engine/index.js:116:18)
      at Web3ProviderEngine._handleAsync (node_modules/web3-provider-engine/index.js:103:3)
      at Timeout._onTimeout (node_modules/web3-provider-engine/index.js:87:12)