message scrolling: Avoid zero-pixel animations.
message scrolling: Avoid zero-pixel animations.

In a recent commit we allowed for `scroll_amount`
to be zero (as an indirect consequence of letting
`scroll_limit` be zero without early exiting).

See 0f75be3e8e317e38f08e1c81fd83ff02c870e9c9

We want to short circuit the call to
`system_initiated_animate_scroll`, partly to save
unnecessary computation, but in particular to avoid
invoking the suppress-pointer-update logic.

hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,tommyip/zulip,kou/zulip,dhcrzf/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,rht/zulip,rishig/zulip,punchagan/zulip,dhcrzf/zulip,rht/zulip,showell/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,tommyip/zulip,rishig/zulip,hackerkid/zulip,brainwane/zulip,timabbott/zulip,rishig/zulip,shubhamdhama/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,rishig/zulip,kou/zulip,synicalsyntax/zulip,shubhamdhama/zulip,andersk/zulip,synicalsyntax/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,kou/zulip,shubhamdhama/zulip,andersk/zulip,eeshangarg/zulip,timabbott/zulip,zulip/zulip,kou/zulip,zulip/zulip,timabbott/zulip,zulip/zulip,showell/zulip,timabbott/zulip,eeshangarg/zulip,dhcrzf/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,timabbott/zulip,rishig/zulip,timabbott/zulip,dhcrzf/zulip,brainwane/zulip,brainwane/zulip,showell/zulip,rishig/zulip,brainwane/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,rht/zulip,showell/zulip,dhcrzf/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,tommyip/zulip,dhcrzf/zulip,rishig/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,dhcrzf/zulip,showell/zulip,hackerkid/zulip,tommyip/zulip,kou/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,rht/zulip,brainwane/zulip,tommyip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,punchagan/zulip,tommyip/zulip,tommyip/zulip,andersk/zulip,synicalsyntax/zulip,shubhamdhama/zulip