MDEV-18205 Assertion `str_length < len' failed in Binary_string::realloc_raw

Bug was introduced in this commit:

  commit: a9ca819897f5c82582bfd3fedb09c78131cf8e00
      Call alloc() instead of realloc()
      Use alloc() if we don't need original string (avoid copy)
      Removed not needed test of str_length in sql_string.cc

copy_if_not_alloced() was forgotten when changing realloc()'s to alloc()'s.
Changing it now.

mysql-test/main/func_str.result

@@ -4961,3 +4961,15 @@ DROP TABLE t1;
 #
 # End of 10.3 tests
 #
+#
+# Start of 10.4 tests
+#
+#
+# MDEV-18205 Assertion `str_length < len' failed in Binary_string::realloc_raw
+#
+SELECT GROUP_CONCAT( UpdateXML( '<a>new year</a>', '/a', '2019-01-01 00:00:00' ), ENCODE('text','pass') ) AS f;
+f
+2019-01-01 00:00:00F}^i
+#
+# End of 10.4 tests
+#

mysql-test/main/func_str.test

@@ -1951,3 +1951,19 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.3 tests
 --echo #
+
+
+--echo #
+--echo # Start of 10.4 tests
+--echo #
+
+--echo #
+--echo # MDEV-18205 Assertion `str_length < len' failed in Binary_string::realloc_raw
+--echo #
+
+SELECT GROUP_CONCAT( UpdateXML( '<a>new year</a>', '/a', '2019-01-01 00:00:00' ), ENCODE('text','pass') ) AS f;
+
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

sql/sql_string.cc

@@ -941,7 +941,7 @@ String *copy_if_not_alloced(String *to,String *from,uint32 from_length)
     (void) from->realloc(from_length);
     return from;
   }
-  if (to->realloc(from_length))
+  if (to->alloc(from_length))
     return from;				// Actually an error
   if ((to->str_length=MY_MIN(from->str_length,from_length)))
     memcpy(to->Ptr,from->Ptr,to->str_length);