runtime: avoid unnecessary scanblock calls

This is the scanstack analog of CL 104737,
which made a similar change for copystack.

name         old time/op  new time/op  delta
ScanStack-8  41.1ms ± 6%  38.9ms ± 5%  -5.52%  (p=0.000 n=50+48)

Change-Id: I7427151dea2895ed3934f8a0f61d96b568019217
Reviewed-on: https://go-review.googlesource.com/105536
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgcmark.go

@@ -839,10 +839,12 @@ func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) {
 			print("runtime: pcdata is ", pcdata, " and ", stkmap.n, " locals stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n")
 			throw("scanframe: bad symbol table")
 		}
+		if stkmap.nbit > 0 {
 			bv := stackmapdata(stkmap, pcdata)
 			size = uintptr(bv.n) * sys.PtrSize
 			scanblock(frame.varp-size, size, bv.bytedata, gcw)
 		}
+	}
 
 	// Scan arguments.
 	if frame.arglen > 0 {
@@ -862,8 +864,10 @@ func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) {
 			}
 			bv = stackmapdata(stkmap, pcdata)
 		}
+		if bv.n > 0 {
 			scanblock(frame.argp, uintptr(bv.n)*sys.PtrSize, bv.bytedata, gcw)
 		}
+	}
 }
 
 type gcDrainFlags int