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/item_strfunc.cc

@@ -603,7 +603,7 @@ String *Item_func_concat::val_str(String *str)
     goto null;
 
   if (res != str)
-    str->copy(res->ptr(), res->length(), res->charset());
+    str->copy_or_move(res->ptr(), res->length(), res->charset());
 
   for (uint i= 1 ; i < arg_count ; i++)
   {

sql/sql_string.cc

@@ -184,9 +184,9 @@ bool String::copy(const String &str)
 
 bool String::copy(const char *str,size_t arg_length, CHARSET_INFO *cs)
 {
+  DBUG_ASSERT(arg_length < UINT_MAX32);
   if (alloc(arg_length))
     return TRUE;
-  DBUG_ASSERT(arg_length < UINT_MAX32);
   if (Ptr == str && arg_length == uint32(str_length))
   {
     /*
@@ -203,6 +203,24 @@ bool String::copy(const char *str,size_t arg_length, CHARSET_INFO *cs)
   return FALSE;
 }
 
+/*
+  Copy string, where strings may overlap.
+  Same as String::copy, but use memmove instead of memcpy to avoid warnings
+  from valgrind
+*/
+
+bool String::copy_or_move(const char *str,size_t arg_length, CHARSET_INFO *cs)
+{
+  DBUG_ASSERT(arg_length < UINT_MAX32);
+  if (alloc(arg_length))
+    return TRUE;
+  if ((str_length=uint32(arg_length)))
+    memmove(Ptr,str,arg_length);
+  Ptr[arg_length]=0;
+  str_charset=cs;
+  return FALSE;
+}
+
 
 /*
   Checks that the source string can be just copied to the destination string
@@ -336,8 +354,9 @@ bool String::set_or_copy_aligned(const char *str,uint32 arg_length,
   /* How many bytes are in incomplete character */
   uint32 offset= (arg_length % cs->mbminlen); 
   
-  if (!offset) /* All characters are complete, just copy */
+  if (!offset)
   {
+    /* All characters are complete, just use given string */
     set(str, arg_length, cs);
     return FALSE;
   }

sql/sql_string.h

@@ -429,6 +429,7 @@ class String
   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);