Support result.free

[kamipo]

Fixes #690.

When calling mysql_stmt_execute, we should not call mysql_stmt_close
before calling mysql_stmt_free_result.

Example:

require 'mysql2'

client = Mysql2::Client.new(
  host: 'localhost',
  username: 'root',
  database: 'mysql',
)

stmt = client.prepare('SELECT 1 FROM dual WHERE 0 = ?')
stmt.execute(1).tap { |result|
  # should call result.free before calling stmt.close
  result.free if result.respond_to?(:free)
}

stmt.close
# Segmentation fault
GC.start

[sodabrew]

I don't want to put the user in a position that a failure to call result.free will crash. How about keeping track of the most recent Result in the Statement object, and if a new statement.execute or Statement object is GCd, then it'll free the last Result at that time?

[sodabrew]

Oh, nice catch on this.

[sodabrew]

Closing in favor of #697 - but if Result#free turns out to be required for the ActiveRecord adapter, then we'll follow up on this.

[sodabrew]

I was wrong, I think we need this. Could you rebase the commits up to current master?