[fix](mysql)fix mysql row buf (#38145)

## Proposed changes
backport: #37936
Issue Number: close #xxx

<!--Describe your changes.-->

be/src/util/mysql_row_buffer.cpp

@@ -44,6 +44,9 @@ namespace doris {
 static uint8_t NEXT_TWO_BYTE = 252;
 static uint8_t NEXT_THREE_BYTE = 253;
 static uint8_t NEXT_EIGHT_BYTE = 254;
+// the EXTRA_RESERVE_BYTE wanner to make sure _pos pointer is always in _buf memory
+// used in reserve() for allocate current buffer
+static size_t EXTRA_RESERVE_BYTE = 16;
 
 // the first byte:
 // <= 250: length
@@ -133,7 +136,7 @@ int MysqlRowBuffer<is_binary_format>::reserve(int64_t size) {
         return 0;
     }
 
-    int64_t alloc_size = std::max(need_size, _buf_size * 2);
+    int64_t alloc_size = std::max(need_size, _buf_size * 2) + EXTRA_RESERVE_BYTE;
     char* new_buf = new char[alloc_size];
 
     size_t offset = _pos - _buf;