check if input file exists

git-svn-id: http://tesseract-ocr.googlecode.com/svn/trunk@1108 d0cd1f9f-072b-0410-8dd7-cf729c803f20
fix C-API to resent C++ API changes - thanks to Nick White

git-svn-id: http://tesseract-ocr.googlecode.com/svn/trunk@1109 d0cd1f9f-072b-0410-8dd7-cf729c803f20