Fix a panic caused by accessing unassigned pointer

This bug was introduced with the fix for checkBlockForHidden votes. One of the
consistency checks fails if the template is uninitialized. The unitialized
pointer is now caught and the function will safely return.

blockmanager.go

@@ -946,7 +946,6 @@ func (b *blockManager) checkBlockForHiddenVotes(block *dcrutil.Block) {
 	// the parent template hasn't yet been updated, so we may
 	// need to use the current template.
 	var template *BlockTemplate
-
 	if b.cachedCurrentTemplate != nil {
 		if b.cachedCurrentTemplate.height ==
 			block.Height() {
@@ -961,6 +960,11 @@ func (b *blockManager) checkBlockForHiddenVotes(block *dcrutil.Block) {
 		}
 	}
 
+	// No template to alter.
+	if template == nil {
+		return
+	}
+
 	// Make sure that the template has the same parent
 	// as the new block.
 	if template.block.Header.PrevBlock !=
@@ -1011,9 +1015,6 @@ func (b *blockManager) checkBlockForHiddenVotes(block *dcrutil.Block) {
 				newVotes = append(newVotes, vote)
 			}
 		}
-	} else {
-		// We have no template, so nothing to update.
-		return
 	}
 
 	// Check the length of the reconstructed voter list for