cmd/trace: fix a few bugs found by staticcheck

First, the regions sort was buggy, as its last comparison was
ineffective.

Second, the insyscall and insyscallRuntime fields were unsigned, so the
check for them being negative was pointless. Make them signed instead,
to also prevent the possibility of underflows when decreasing numbers
that might realistically be 0.

Third, the color constants were all untyped strings except the first
one. Be consistent with their typing.

Change-Id: I4eb8d08028ed92589493c2a4b9cc5a88d83f769b
Reviewed-on: https://go-review.googlesource.com/113895


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>