Bug #31591391: FIX BASE BUG IN 5.7/5.6 TOO, OVERFLOW IN FIELD_ENUM::PACK()

Backport the fix for bug #29948029 (ASSERTION FAILURE IN
__GNU_CXX::NEW_ALLOCATOR() AT EXT/NEW_ALLOCATOR.H) to 5.6 and 5.7.

Change-Id: I1397de18a1932724facd1a974c56cbada25b9b25

Author: Steinar H. Gunderson

sql/field.cc

@@ -1,5 +1,5 @@
 /*
-   Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2020, Oracle and/or its affiliates.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2.0,
@@ -8857,7 +8857,7 @@ uchar *Field_enum::pack(uchar *to, const uchar *from,
   switch (packlength)
   {
   case 1:
-    *to = *from;
+    if (max_length > 0) *to = *from;
     DBUG_RETURN(to + 1);
   case 2: DBUG_RETURN(pack_int16(to, from, low_byte_first));
   case 3: DBUG_RETURN(pack_int24(to, from, low_byte_first));

sql/field.h

@@ -1,7 +1,7 @@
 #ifndef FIELD_INCLUDED
 #define FIELD_INCLUDED
 
-/* Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2020, Oracle and/or its affiliates.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2.0,
@@ -1746,7 +1746,7 @@ class Field_tiny :public Field_num {
   virtual uchar *pack(uchar* to, const uchar *from,
                       uint max_length, bool low_byte_first)
   {
-    *to= *from;
+    if (max_length > 0) *to= *from;
     return to + 1;
   }