Ghostscript CVE-2020-15900: Integer Overflow in PostScript Calculator bitshift Operator

The PostScript Type 4 Calculator function in Ghostscript (versions through 9.52) contains an integer overflow vulnerability in the bitshift operator (PtCr_bitshift). The bounds check for the shift operand is off-by-one, allowing shift amounts that exceed the safe range for signed integers. This causes undefined behavior when processing specially crafted PostScript calculator functions in PDF documents, potentially leading to memory corruption, heap overflow, or code execution.