runtime: Fix bug due to elided return.

A previous change to mbitmap.go dropped a return on a
path the seems not to be excersized. This was a mistake that
this CL fixes.

Change-Id: I715ee4ef08f5bf8d9f53cee84e8fb31a237e2d43
Reviewed-on: https://go-review.googlesource.com/9295Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -187,6 +187,7 @@ func heapBitsForObject(p uintptr) (base uintptr, hbits heapBits, s *mspan) {
 			printunlock()
 			throw("objectstart: bad pointer in unexpected span")
 		}
+		return
 	}
 	// If this span holds object of a power of 2 size, just mask off the bits to
 	// the interior of the object. Otherwise use the size to get the base.