src: don't call DecodeWrite() on Buffers

Don't call DecodeWrite() with a Buffer as its argument because it in
turn calls StringBytes::Write() and that method expects a Local<String>.

"Why then does that function take a Local<Value>?" I hear you ask.
Good question but I don't have the answer.  I added a CHECK for good
measure and what do you know, all of a sudden a large number of crypto
tests started failing.

Calling DecodeWrite(BINARY) on a buffer is nonsensical anyway: if you
want the contents of the buffer, just copy out the data, there is no
need to decode it - and that's exactly what this commit does.

Fixes a great many instances of the following run-time error in debug
builds:

    FATAL ERROR: v8::String::Cast() Could not convert to string

src/node_crypto.cc

@@ -764,20 +764,9 @@ void SecureContext::LoadPKCS12(const FunctionCallbackInfo<Value>& args) {
 
   if (args.Length() >= 2) {
     ASSERT_IS_BUFFER(args[1]);
-
-    int passlen = Buffer::Length(args[1]);
-    if (passlen < 0) {
-      BIO_free_all(in);
-      return env->ThrowTypeError("Bad password");
-    }
+    size_t passlen = Buffer::Length(args[1]);
     pass = new char[passlen + 1];
-    int pass_written = DecodeWrite(env->isolate(),
-                                   pass,
-                                   passlen,
-                                   args[1],
-                                   BINARY);
-
-    assert(pass_written == passlen);
+    memcpy(pass, Buffer::Data(args[1]), passlen);
     pass[passlen] = '\0';
   }
 
@@ -1167,18 +1156,12 @@ void SSLWrap<Base>::SetSession(const FunctionCallbackInfo<Value>& args) {
   }
 
   ASSERT_IS_BUFFER(args[0]);
-  ssize_t slen = Buffer::Length(args[0]);
-
-  if (slen < 0)
-    return env->ThrowTypeError("Bad argument");
-
+  size_t slen = Buffer::Length(args[0]);
   char* sbuf = new char[slen];
-
-  ssize_t wlen = DecodeWrite(env->isolate(), sbuf, slen, args[0], BINARY);
-  assert(wlen == slen);
+  memcpy(sbuf, Buffer::Data(args[0]), slen);
 
   const unsigned char* p = reinterpret_cast<const unsigned char*>(sbuf);
-  SSL_SESSION* sess = d2i_SSL_SESSION(NULL, &p, wlen);
+  SSL_SESSION* sess = d2i_SSL_SESSION(NULL, &p, slen);
 
   delete[] sbuf;
 
@@ -3810,8 +3793,6 @@ void PBKDF2(const FunctionCallbackInfo<Value>& args) {
   ssize_t passlen = -1;
   ssize_t saltlen = -1;
   ssize_t keylen = -1;
-  ssize_t pass_written = -1;
-  ssize_t salt_written = -1;
   ssize_t iter = -1;
   PBKDF2Request* req = NULL;
   Local<Object> obj;
@@ -3832,8 +3813,7 @@ void PBKDF2(const FunctionCallbackInfo<Value>& args) {
   if (pass == NULL) {
     FatalError("node::PBKDF2()", "Out of Memory");
   }
-  pass_written = DecodeWrite(env->isolate(), pass, passlen, args[0], BINARY);
-  assert(pass_written == passlen);
+  memcpy(pass, Buffer::Data(args[0]), passlen);
 
   ASSERT_IS_BUFFER(args[1]);
   saltlen = Buffer::Length(args[1]);
@@ -3846,8 +3826,7 @@ void PBKDF2(const FunctionCallbackInfo<Value>& args) {
   if (salt == NULL) {
     FatalError("node::PBKDF2()", "Out of Memory");
   }
-  salt_written = DecodeWrite(env->isolate(), salt, saltlen, args[1], BINARY);
-  assert(salt_written == saltlen);
+  memcpy(salt, Buffer::Data(args[1]), saltlen);
 
   if (!args[2]->IsNumber()) {
     type_error = "Iterations not a number";

src/string_bytes.cc

@@ -301,6 +301,7 @@ size_t StringBytes::Write(Isolate* isolate,
   size_t len = 0;
   bool is_extern = GetExternalParts(isolate, val, &data, &len);
 
+  CHECK(val->IsString() == true);
   Local<String> str = val.As<String>();
   len = len < buflen ? len : buflen;