... (valgrind reports Invalid read)

Add gimp_babl_is_valid(), which takes a GimpImageBaseType and a
GimpPrecision, and determines whether the image-type/precision
combination is valid.  Use this function to validate that loaded
XCFs use a valid type/precision combination, before trying to
create the image.  Otherwise, we get a CRITICAL, and eventually a
segfault, when the combination is invalid.

Use the same function to validate the arguments of
gimp_image_new().

(cherry picked from commit a0a62656)