cairo_pdf_version_to_string: Check for negative values
Before this commit, cairo_pdf_version_to_string() would return the result of an out-of-bounds array access when called with a negative value. This commit adds a check against this.
No unit test added since there are no tests for cairo_pdf_version_to_string() that I could easily add such a test to.
Fixes: #590 (closed) Signed-off-by: Uli Schlachter psychon@znc.in