[wasm] Fix EnsureSpace in the ZoneBuffer of the wasm encoder.

src/wasm/encoder.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_WASM_ENCODER_H_
 #define V8_WASM_ENCODER_H_
 
 #include "src/signature.h"
 #include "src/zone-containers.h"
 
@@ skipping 72 matching lines @@
     }
   }
 
   size_t offset() { return static_cast<size_t>(pos_ - buffer_); }
   size_t size() { return static_cast<size_t>(pos_ - buffer_); }
   const byte* begin() { return buffer_; }
   const byte* end() { return pos_; }
 
   void EnsureSpace(size_t size) {
     if ((pos_ + size) > end_) {
-      size_t new_size = 4096 + (end_ - buffer_) * 3;
+      size_t new_size = 4096 + size + (end_ - buffer_) * 3;
       byte* new_buffer = reinterpret_cast<byte*>(zone_->New(new_size));
       memcpy(new_buffer, buffer_, (pos_ - buffer_));
       pos_ = new_buffer + (pos_ - buffer_);
       buffer_ = new_buffer;
       end_ = new_buffer + new_size;
     }
+    DCHECK(pos_ + size <= end_);
   }
 
   byte** pos_ptr() { return &pos_; }
 
  private:
   Zone* zone_;
   byte* buffer_;
   byte* pos_;
   byte* end_;
 };
@@ skipping 130 matching lines @@
   ZoneVector<std::pair<LocalType, bool>> globals_;
   SignatureMap signature_map_;
   int start_function_index_;
 };
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_WASM_ENCODER_H_