runtime: revise CL 105140044 (defer nil) to work on Windows

It appears that something about Go on Windows
cannot handle the fault cause by a jump to address 0.
The way Go represents and calls functions, this
never happened at all, until CL 105140044.

This CL changes the code added in CL 105140044
to make jump to 0 impossible once again.

Fixes #8047. (again, on Windows)

TBR=bradfitz
R=golang-codereviews, dave
CC=adg, golang-codereviews, iant, r
https://golang.org/cl/105120044

src/pkg/runtime/stack.c

@@ -9,6 +9,7 @@
 #include "funcdata.h"
 #include "typekind.h"
 #include "type.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
@@ -851,6 +852,13 @@ runtime·newstack(void)
 	*(int32*)345 = 123;	// never return
 }
 
+#pragma textflag NOSPLIT
+void
+runtime·nilfunc(void)
+{
+	*(byte*)0 = 0;
+}
+
 // adjust Gobuf as if it executed a call to fn
 // and then did an immediate gosave.
 void
@@ -858,9 +866,10 @@ runtime·gostartcallfn(Gobuf *gobuf, FuncVal *fv)
 {
 	void *fn;
 
-	fn = nil;
 	if(fv != nil)
 		fn = fv->fn;
+	else
+		fn = runtime·nilfunc;
 	runtime·gostartcall(gobuf, fn, fv);
 }