stored: fix unintended decrement of block_num

When bsr() is called to initiare a re-read on a tape, it will decrement
ths block_num. As during the re-read the block_num was not incremented,
it shouldn't be decremented.

core/src/stored/block.cc

@@ -1060,7 +1060,9 @@ DeviceControlRecord::ReadStatus DeviceControlRecord::ReadBlockFromDev(bool)
     // Attempt to Reposition to re-read the block
     if (dev->IsTape()) {
       Dmsg0(250, "BootStrapRecord for reread; block too big for buffer.\n");
-      if (!dev->bsr(1)) {
+      if (dev->bsr(1)) {
+        dev->block_num++;  // reincrement what bsr() decremented
+      } else {
         Mmsg(dev->errmsg, "%s", dev->bstrerror());
         Jmsg(jcr, M_ERROR, 0, "%s", dev->errmsg);
         block->read_len = 0;