Bug#45288: pb2 returns a lot of compilation warnings on linux

Fix warnings flagged by the new warning option -Wunused-but-set-variable
that was added to GCC 4.6 and that is enabled by -Wunused and -Wall. The
option causes a warning whenever a local variable is assigned to but is
later unused. It also warns about meaningless pointer dereferences.

client/mysql.cc:
  Meaningless pointer dereferences.
client/mysql_upgrade.c:
  Check whether reading from the file succeeded.
extra/comp_err.c:
  Unused.
extra/yassl/src/yassl_imp.cpp:
  Skip instead of reading data that is discarded.
include/my_pthread.h:
  Variable is only used in debug builds.
include/mysys_err.h:
  Add new error messages.
mysys/errors.c:
  Add new error message for permission related functions.
mysys/mf_iocache.c:
  Variable is only checked under THREAD.
mysys/my_copy.c:
  Raise a error if chmod or chown fails.
mysys/my_redel.c:
  Raise a error if chmod or chown fails.
regex/engine.c:
  Use a equivalent variable for the assert.
server-tools/instance-manager/instance_options.cc:
  Unused.
sql/field.cc:
  Unused.
sql/item.cc:
  Unused.
sql/log.cc:
  Do not ignore the return value of freopen: only set buffer if
  reopening succeeds.
  
  Adjust doxygen comment to the right function.
  
  Pass message lenght to log function.
sql/mysqld.cc:
  Do not ignore the return value of freopen: only set buffer if
  reopening succeeds.
sql/partition_info.cc:
  Unused.
sql/slave.cc:
  No need to set pointer to the address of '\0'.
sql/spatial.cc:
  Unused. Left for historical purposes.
sql/sql_acl.cc:
  Unused.
sql/sql_base.cc:
  Pointers are always set to the same variables.
sql/sql_parse.cc:
  End statement if reading fails.
  
  Store the buffer after it has actually been updated.
sql/sql_repl.cc:
  No need to set pointer to the address of '\0'.
sql/sql_show.cc:
  Put variable under the same ifdef block.
sql/udf_example.c:
  Set null pointer flag appropriately.
storage/csv/ha_tina.cc:
  Meaningless dereferences.
storage/example/ha_example.cc:
  Return the error since it's available.
storage/myisam/mi_locking.c:
  Remove unused and dead code.

client/mysql.cc

@@ -3639,7 +3639,7 @@ xmlencode_print(const char *src, uint length)
     tee_fputs("NULL", PAGER);
   else
   {
-    for (const char *p = src; length; *p++, length--)
+    for (const char *p = src; length; p++, length--)
     {
       const char *t;
       if ((t = array_value(xmlmeta, *p)))
@@ -4726,7 +4726,7 @@ static const char* construct_prompt()
   struct tm *t = localtime(&lclock);
 
   /* parse thru the settings for the prompt */
-  for (char *c = current_prompt; *c ; *c++)
+  for (char *c = current_prompt; *c ; c++)
   {
     if (*c != PROMPT_CHAR)
 	processed_prompt.append(*c);