Add a null pointer check before getting the family name of the given color space in CPDF_ColorSpace::Load

The test file defines a wrong color space object (7 0 obj). In the content of 7 0 obj,
the reserved obj (0 0 R) is used. The process of loading color space returns NULL when
the reserved obj (0 0 R) is found. For the error color space, it only needs to return
NULL when an error is detected.

BUG=403032
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/477413002