Fixed compiler warnings.

In set_var.cc, the code was not properly returning an error code
if close_cached_tables() failed.

In sql_tables.cc, the code was not returning properly an error code
if lock_table_names() failed.

Both cases are bugs, introduced in 5.1 only by recent changes.


sql/set_var.cc:
  Fixed build warnings:
  set_var.cc:3861: warning: 'result' may be used uninitialized in this function
sql/sql_table.cc:
  Fixed build warnings:
  sql_table.cc:5307: warning: 'error' may be used uninitialized in this function