Fix several wasm warnings an a use after free.

src/wasm/module-decoder.cc

 // 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.
 
 #include "src/macro-assembler.h"
 #include "src/objects.h"
 #include "src/v8.h"
 
 #include "src/wasm/decoder.h"
 #include "src/wasm/module-decoder.h"
@@ skipping 353 matching lines @@
 
     TreeResult result =
         VerifyWasmCode(&fenv, start_, start_ + function->code_start_offset,
                        start_ + function->code_end_offset);
     if (result.failed()) {
       // Wrap the error message from the function decoder.
       std::ostringstream str;
       str << "in function #" << func_num << ": ";
       // TODO(titzer): add function name for the user?
       str << result;
-      const char* raw = str.str().c_str();
+      std::string strval = str.str();
+      const char* raw = strval.c_str();
       size_t len = strlen(raw);
       char* buffer = new char[len];
       strncpy(buffer, raw, len);
       buffer[len - 1] = 0;
 
       // Copy error code and location.
       result_.CopyFrom(result);
       result_.error_msg.Reset(buffer);
     }
   }
 
   // Reads a single 32-bit unsigned integer interpreted as an offset, checking
   // the offset is within bounds and advances.
   uint32_t offset(const char* name = nullptr) {
     uint32_t offset = u32(name ? name : "offset");
-    if (offset > (limit_ - start_)) {
+    if (offset > static_cast<uint32_t>(limit_ - start_)) {
       error(pc_ - sizeof(uint32_t), "offset out of bounds of module");
     }
     return offset;
   }
 
   // Reads a single 32-bit unsigned integer interpreted as an offset into the
   // data and validating the string there and advances.
   uint32_t string(const char* name = nullptr) {
     return offset(name ? name : "string");  // TODO(titzer): validate string
   }
@@ skipping 122 matching lines @@
   if (function_start > function_end) return FunctionError("start > end");
   if (size > kMaxFunctionSize)
     return FunctionError("size > maximum function size");
   WasmFunction* function = new WasmFunction();
   ModuleDecoder decoder(zone, function_start, function_end, false);
   return decoder.DecodeSingleFunction(module_env, function);
 }
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8