runtime: fix integer comparison in signal handling

(sig is unsigned, so sig-1 >= 0 is always true.)

Fixes #11281.

Change-Id: I4b9d784da6e3cc80816f2d2f7228d5d8a237e2d5
Reviewed-on: https://go-review.googlesource.com/17457Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/signal1_unix.go

@@ -228,11 +228,11 @@ func ensureSigM() {
 		for {
 			select {
 			case sig := <-enableSigChan:
-				if b := sig - 1; b >= 0 {
+				if b := sig - 1; sig > 0 {
 					sigBlocked[b/32] &^= (1 << (b & 31))
 				}
 			case sig := <-disableSigChan:
-				if b := sig - 1; b >= 0 {
+				if b := sig - 1; sig > 0 {
 					sigBlocked[b/32] |= (1 << (b & 31))
 				}
 			}