Block page: check gas limit value before division

[vbaranov]

Motivation

An internal error on the block pages when Blockscout connected to Aurora JSON RPC.

2021-09-23T18:07:34.632 [error] GenServer #PID<0.2260.0> terminating
** (ArithmeticError) bad argument in arithmetic expression
    (block_scout_web 0.0.1) lib/block_scout_web/templates/block/_tile.html.eex:71: BlockScoutWeb.BlockView."_tile.html"/1
    (phoenix 1.5.6) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
    (phoenix 1.5.6) lib/phoenix/view.ex:479: Phoenix.View.render_to_string/3
    (block_scout_web 0.0.1) lib/block_scout_web/channels/block_channel.ex:24: BlockScoutWeb.BlockChannel.handle_out/3
    (phoenix 1.5.6) lib/phoenix/channel/server.ex:328: Phoenix.Channel.Server.handle_info/2
    (stdlib 3.15.1) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.15.1) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.15.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Changelog

Check gas limit value before division

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools