Fix bug in PGRAstroDeclString.

Changed the comparison with zero from dms.degrees to decl. When decl is negative but its absolute value is less than 1, dms.degrees could be non-negative, while dms.minutes and dms.seconds would be negative. Just comparing dms.degrees would lead to a small positive angle being reported in the calculated string, rather than a small negative angle. Comparing instead to decl removes this potential.