cmd/compile: don't crash on -d=ssa/

I forgot how to pull up the ssa debug options help, so instead of
writing -d=ssa/help, I just wrote -d=ssa/. Much to my amusement, the
compiler just crashed, as shown below. Fix that.

	panic: runtime error: index out of range

	goroutine 1 [running]:
	cmd/compile/internal/ssa.PhaseOption(0x7ffc375d2b70, 0x0, 0xdbff91, 0x5, 0x1, 0x0, 0x0, 0x1, 0x1)
	    /home/mvdan/tip/src/cmd/compile/internal/ssa/compile.go:327 +0x1876
	cmd/compile/internal/gc.Main(0xde7bd8)
	    /home/mvdan/tip/src/cmd/compile/internal/gc/main.go:411 +0x41d0
	main.main()
	    /home/mvdan/tip/src/cmd/compile/main.go:51 +0xab

Change-Id: Ia2ad394382ddf8f4498b16b5cfb49be0317fc1aa
Reviewed-on: https://go-review.googlesource.com/c/154421


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>