Remove valgrind warnings from Item_str_concat

This warning come from a copy() operation of type:
memcpy(ptr, ptr+A, B), which is safe but produces a warning
when run with valgrind.

To avoid the warning, I added copy_or_move() method which uses
memmove() instead of memcpy().

In 10.3 the change in item_strfunc::Item_func_concat() has to be mirroed
in Item_func_concat_oracle() to avoid future valgrind warnings.

sql/sql_string.h

@@ -429,6 +429,7 @@ public:
   bool copy();					// Alloc string if not alloced
   bool copy(const String &s);			// Allocate new string
   bool copy(const char *s,size_t arg_length, CHARSET_INFO *cs);	// Allocate new string
+  bool copy_or_move(const char *s,size_t arg_length, CHARSET_INFO *cs);
   static bool needs_conversion(uint32 arg_length,
   			       CHARSET_INFO *cs_from, CHARSET_INFO *cs_to,
 			       uint32 *offset);