BUG#20839 Illegal error code: 155 returned downgrading from 5.1.12-> 5.1.11

post-review fixups - magnus suggested creating dynstr_trunc instead of doing
it manually.

client/mysqldump.c

@@ -2783,7 +2783,7 @@ static int dump_tablespaces_for_tables(char *db, char **table_names, int tables)
     dynstr_append(&where, name_buff);
     dynstr_append(&where, "',");
   }
-  where.str[--where.length]= '\0';
+  dynstr_trunc(&where, 1);
   dynstr_append(&where,"))");
 
   DBUG_PRINT("info",("Dump TS for Tables where: %s",where));
@@ -2813,7 +2813,7 @@ static int dump_tablespaces_for_databases(char** databases)
     dynstr_append(&where, db_name_buff);
     dynstr_append(&where, "',");
   }
-  where.str[--where.length]='\0';
+  dynstr_trunc(&where, 1);
   dynstr_append(&where,"))");
 
   DBUG_PRINT("info",("Dump TS for DBs where: %s",where));

include/my_sys.h

@@ -778,6 +778,7 @@ my_bool dynstr_append_mem(DYNAMIC_STRING *str, const char *append,
 			  uint length);
 extern my_bool dynstr_set(DYNAMIC_STRING *str, const char *init_str);
 extern my_bool dynstr_realloc(DYNAMIC_STRING *str, ulong additional_size);
+extern my_bool dynstr_trunc(DYNAMIC_STRING *str, int n);
 extern void dynstr_free(DYNAMIC_STRING *str);
 #ifdef HAVE_MLOCK
 extern byte *my_malloc_lock(uint length,myf flags);

mysys/string.c

@@ -115,6 +115,12 @@ my_bool dynstr_append_mem(DYNAMIC_STRING *str, const char *append,
   return FALSE;
 }
 
+my_bool dynstr_trunc(DYNAMIC_STRING *str, int n)
+{
+  str->length-=n;
+  str->str[str->length]= '\0';
+  return FALSE;
+}
 
 void dynstr_free(DYNAMIC_STRING *str)
 {