src: fix potential memory leak on early return

src/node_crypto.cc

@@ -3009,9 +3009,6 @@ class Sign : public ObjectWrap {
     unsigned int md_len;
     Local<Value> outString;
 
-    md_len = 8192; // Maximum key size is 8192 bits
-    md_value = new unsigned char[md_len];
-
     ASSERT_IS_BUFFER(args[0]);
     ssize_t len = Buffer::Length(args[0]);
 
@@ -3019,6 +3016,9 @@ class Sign : public ObjectWrap {
     ssize_t written = DecodeWrite(buf, len, args[0], BUFFER);
     assert(written == len);
 
+    md_len = 8192; // Maximum key size is 8192 bits
+    md_value = new unsigned char[md_len];
+
     int r = sign->SignFinal(&md_value, &md_len, buf, len);
     if (r == 0) {
       md_value = NULL;