Fix gcc7 compile

mysql.cc: In function 'void build_completion_hash(bool, bool)':
mysql.cc:2687:37: error: invalid conversion from 'char' to 'char*' [-fpermissive]
       field_names[i][num_fields*2]= '\0';
                                     ^~~~
Patch was partly backported from upstream commit:
https://github.com/mysql/mysql-server/commit/ae21683d980d5fe9e39bd0193827ea3604256eb9

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: add more gcc 7.x fixes in instance_map.cc.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/client/mysql.cc
===================================================================
--- a/client/mysql.cc
+++ b/client/mysql.cc
@@ -2684,7 +2684,7 @@
         mysql_free_result(fields);
         break;
       }
-      field_names[i][num_fields*2]= '\0';
+      field_names[i][num_fields*2]= NULL;
       j=0;
       while ((sql_field=mysql_fetch_field(fields)))
       {
Index: b/server-tools/instance-manager/instance_map.cc
===================================================================
--- a/server-tools/instance-manager/instance_map.cc
+++ b/server-tools/instance-manager/instance_map.cc
@@ -526,12 +526,12 @@
              Options::Main::config_file);
 
     argv_options[1]= defaults_file_arg;
-    argv_options[2]= '\0';
+    argv_options[2]= NULL;
 
     argc= 2;
   }
   else
-    argv_options[1]= '\0';
+    argv_options[1]= NULL;
 
   /*
     If the routine failed, we'll simply fallback to defaults in