runtime: don't crash if signal handler info argument is nil

Apparently on Solaris 10 a SA_SIGINFO signal handler can be invoked with
    a nil info argument.  I would not have believed it but I've now seen it
    happen, and the sigaction man page actually says "If the second argument
    is not equal to NULL, it points to a siginfo_t structure...."  So, if
    that happens, don't crash.
    
    Also fix another case where we want to make sure that &T{} does not
    allocate.
    
    Reviewed-on: https://go-review.googlesource.com/33150

From-SVN: r242403

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-3c8d91cff0ad3d233ebd268f88a3749d38a0aac1
+eb716b515356166d3177e6244619be5901f31162
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/signal_gccgo.go

@@ -47,7 +47,14 @@ type sigctxt struct {
 	ctxt unsafe.Pointer
 }
 
-func (c *sigctxt) sigcode() uint64 { return uint64(c.info.si_code) }
+func (c *sigctxt) sigcode() uint64 {
+	if c.info == nil {
+		// This can happen on Solaris 10.  We don't know the
+		// code, just avoid a misleading value.
+		return _SI_USER + 1
+	}
+	return uint64(c.info.si_code)
+}
 
 //go:nosplit
 func sigblock() {

libgo/go/runtime/signal_sigtramp.go

@@ -29,7 +29,8 @@ func sigtrampgo(sig uint32, info *_siginfo_t, ctx unsafe.Pointer) {
 			// get here anyhow.
 			return
 		}
-		badsignal(uintptr(sig), &sigctxt{info, ctx})
+		c := sigctxt{info, ctx}
+		badsignal(uintptr(sig), &c)
 		return
 	}
 

libgo/runtime/go-signal.c

@@ -187,7 +187,11 @@ getSiginfo(siginfo_t *info, void *context __attribute__((unused)))
 	Location loc[1];
 	int32 n;
 
-	ret.sigaddr = (uintptr)(info->si_addr);
+	if (info == nil) {
+		ret.sigaddr = 0;
+	} else {
+		ret.sigaddr = (uintptr)(info->si_addr);
+	}
 	ret.sigpc = 0;
 
 	// There doesn't seem to be a portable way to get the PC.