cmd/compile: zero all three argstorage slots

These changes were missed when going from 2 to 3 argstorage slots.
https://go-review.googlesource.com/20296/

Change-Id: I930a307bb0b695bf1ae088030c9bbb6d14ca31d2
Reviewed-on: https://go-review.googlesource.com/21841
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index fd4eb64..6c364ad 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -185,6 +185,7 @@
 	}
 	v.argstorage[0] = nil
 	v.argstorage[1] = nil
+	v.argstorage[2] = nil
 	v.Args = v.argstorage[:0]
 }