libcolord: Return the correct result from cd_mat33_matrix_multiply()

If @mat_dest was allocated on the stack then it was never cleared and relied on
the undefined behaviour of the default floating point values in the structure.
trivial: small whitespace fix
libcolord: Initialize the value of the CCMX matrix to avoid undefined values

If we don't call cd_it8_set_matrix() before cd_it8_save_to_file() on a CdIt8
object of type @CD_IT8_KIND_CCMX then we could be writing undefined values.