runtime: fix scanning of not started goroutines

The stack scanner for not started goroutines ignored the arguments
area when its size was unknown.  With this change, the distance
between the stack pointer and the stack base will be used instead.

Fixes #5486

R=golang-dev, bradfitz, iant, dvyukov
CC=golang-dev
https://golang.org/cl/9440043

src/pkg/runtime/mgc0.c

@@ -1454,11 +1454,18 @@ addstackroots(G *gp)
 			// be scanned.  No other live values should be on the
 			// stack.
 			f = runtime·findfunc((uintptr)gp->fnstart->fn);
-			if(f->args > 0) {
+			if(f->args != 0) {
 				if(thechar == '5')
 					sp += sizeof(uintptr);
-				addroot((Obj){sp, f->args, 0});
-			}
+				// If the size of the arguments is known
+				// scan just the incoming arguments.
+				// Otherwise, scan everything between the
+				// top and the bottom of the stack.
+				if(f->args > 0)
+					addroot((Obj){sp, f->args, 0});
+				else
+					addroot((Obj){sp, (byte*)stk - sp, 0}); 
+			} 
 			return;
 		}
 	}