Bugfix Image_Imagemagick->_rounded($radius, $sides, $antialias = 0)

If we use imagemagick drivers , the Controller doesn't work.  It create an error command ,and throw "bash: Unexpected symbol `('Incorrect syntax near".
 Bugfix Image_Imagemagick->_watermark($filename, $position, $padding = 5)

 Undefined variable: $image in the line 124.
Merge pull request #1453 from fastcgi/1.7/develop

Bugfix Image_Imagemagick->_watermark() and _rounded() functions