[wasm] Disassembler: make label printing more robust

The return value of StringBuilder::cursor() isn't safe to store across
print operations, because it will become stale if the StringBuilder
needs to grow its buffer. The solution is to store the length() instead,
and recompute the raw pointer from the updated start() when needed.

Change-Id: Id453e39743644a5df9f7cbb8b1acaea7f5890453
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3782671
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81912}

src/wasm/wasm-disassembler.cc

@@ -290,7 +290,7 @@ class ImmediatesPrinter {
 
   void PrintDepthAsLabel(int imm_depth) {
     out_ << " ";
-    const char* label_start = out_.cursor();
+    size_t label_start_position = out_.length();
     int depth = imm_depth;
     if (owner_->current_opcode_ == kExprDelegate) depth++;
     // Be robust: if the module is invalid, print what we got.
@@ -309,8 +309,8 @@ class ImmediatesPrinter {
     names()->PrintLabelName(out_, owner_->func_index_,
                             label_info.name_section_index,
                             owner_->label_generation_index_++);
-    label_info.length = static_cast<size_t>(out_.cursor() - label_start);
-    owner_->out_->PatchLabel(label_info, label_start);
+    label_info.length = out_.length() - label_start_position;
+    owner_->out_->PatchLabel(label_info, out_.start() + label_start_position);
   }
 
   void BlockType(BlockTypeImmediate<validate>& imm) {