Fix invalid + of std::string and int

These warnings were emitted by clang++, and they are real bugs.
src/rpc/core_rpc_server.cpp:208:58: warning: adding 'uint64_t'
      (aka 'unsigned long') to a string does not append to the string
      [-Wstring-plus-int]
        res.status = "Error retrieving block at height " + height;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
The obvious intent is achieved by using std::to_string().

src/rpc/core_rpc_server.cpp

@@ -205,7 +205,7 @@ namespace cryptonote
       }
       catch (...)
       {
-        res.status = "Error retrieving block at height " + height;
+        res.status = "Error retrieving block at height " + std::to_string(height);
         return true;
       }
       std::list<transaction> txs;

src/wallet/wallet2.cpp

@@ -5424,9 +5424,9 @@ uint64_t wallet2::get_blockchain_height_by_date(uint16_t year, uint8_t month, ui
       throw std::runtime_error(oss.str());
     }
     cryptonote::block blk_min, blk_mid, blk_max;
-    if (!parse_and_validate_block_from_blob(res.blocks[0].block, blk_min)) throw std::runtime_error("failed to parse blob at height " + height_min);
-    if (!parse_and_validate_block_from_blob(res.blocks[1].block, blk_mid)) throw std::runtime_error("failed to parse blob at height " + height_mid);
-    if (!parse_and_validate_block_from_blob(res.blocks[2].block, blk_max)) throw std::runtime_error("failed to parse blob at height " + height_max);
+    if (!parse_and_validate_block_from_blob(res.blocks[0].block, blk_min)) throw std::runtime_error("failed to parse blob at height " + std::to_string(height_min));
+    if (!parse_and_validate_block_from_blob(res.blocks[1].block, blk_mid)) throw std::runtime_error("failed to parse blob at height " + std::to_string(height_mid));
+    if (!parse_and_validate_block_from_blob(res.blocks[2].block, blk_max)) throw std::runtime_error("failed to parse blob at height " + std::to_string(height_max));
     uint64_t timestamp_min = blk_min.timestamp;
     uint64_t timestamp_mid = blk_mid.timestamp;
     uint64_t timestamp_max = blk_max.timestamp;