runtime: don't stop bitmap dump at BitsDead

Stack bitmaps need to be scanned past any BitsDead entries.

Object bitmaps will not have any BitsDead in them (bitmap extraction stops at
the first BitsDead entry in makeheapobjbv).  data/bss bitmaps also have no BitsDead entries.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/168270043

src/runtime/heapdump.c

@@ -251,7 +251,9 @@ dumpbv(BitVector *bv, uintptr offset)
 	for(i = 0; i < bv->n; i += BitsPerPointer) {
 		switch(bv->bytedata[i/8] >> i%8 & 3) {
 		case BitsDead:
-			return;
+			// BitsDead has already been processed in makeheapobjbv.
+			// We should only see it in stack maps, in which case we should continue processing.
+			break;
 		case BitsScalar:
 			break;
 		case BitsPointer: