6l: correct logic for morestack choice

The frame that gets allocated is for both
the args and the autos.  If together they
exceed the default frame size, we need to
tell morestack about both so that it allocates
a large enough frame.

Sanity check stack pointer in morestack
to catch similar bugs.

R=ken2
CC=golang-dev
https://golang.org/cl/2609041

src/cmd/6l/pass.c

@@ -469,7 +469,7 @@ dostkoff(void)
 
 			/* 160 comes from 3 calls (3*8) 4 safes (4*8) and 104 guard */
 			moreconst1 = 0;
-			if(autoffset+160 > 4096)
+			if(autoffset+160+textarg > 4096)
 				moreconst1 = (autoffset+160) & ~7LL;
 			moreconst2 = textarg;
 

src/pkg/runtime/proc.c

@@ -749,6 +749,8 @@ newstack(void)
 	args = m->moreargs;
 	g1 = m->curg;
 
+	if(m->morebuf.sp < g1->stackguard - StackGuard)
+		throw("split stack overflow");
 
 	if(frame == 1 && args > 0 && m->morebuf.sp - sizeof(Stktop) - args - 32 > g1->stackguard) {
 		// special case: called from reflect.call (frame == 1)