[maglev] Fix colouring empty verticals

Accidentally tried to print the colour '-1' if there was no line colour
set yet but the current colour had changed.

Bug: v8:7700
Change-Id: Iba3cbe51fd0e31e389e614d0a42e59147a51b902
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3859355
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82789}

src/maglev/maglev-graph-printer.cc

@@ -158,7 +158,8 @@ void PrintVerticalArrows(std::ostream& os,
       desired_color = (i % 6) + 1;
       c.AddVertical();
     }
-    if (FLAG_log_colour && desired_color != current_color) {
+    if (FLAG_log_colour && desired_color != current_color &&
+        desired_color != -1) {
       os << "\033[0;3" << desired_color << "m";
       current_color = desired_color;
     }
@@ -343,7 +344,8 @@ void MaglevPrintingVisitor::PreProcessBasicBlock(
         desired_color = (i % 6) + 1;
         c.AddVertical();
       }
-      if (FLAG_log_colour && current_color != desired_color) {
+      if (FLAG_log_colour && current_color != desired_color &&
+          desired_color != -1) {
         os_ << "\033[0;3" << desired_color << "m";
         current_color = desired_color;
       }