[Bug #19004] `Complex.polar` handles complex singular `abs` argument

	`Complex.polar` accepts Complex values as arguments for the polar form as long
	as the value of the complex has no imaginary part (ie it is 'real'). In
	`f_complex_polar` this is handled by extracting the real part of the arguments.
	However in the case `polar` is called with only a single argument, the absolute
	value (abs), then the Complex is created without applying a check on the type
	of abs, meaning it is possible to create a Complex where the real part is itself
	an instance of a Complex. This change removes the short circuit for the single
	argument case meaning the real part extraction is performed correctly
	(by f_complex_polar).

	Also adds an example to `spec/ruby/core/complex/polar_spec.rb` to check that
	the real part of a complex argument is correctly extracted and used in the
	resulting Complex real and imaginary parts.
	---
	 complex.c                            | 13 ++++++-------
	 spec/ruby/core/complex/polar_spec.rb | 16 ++++++++++++++++
	 2 files changed, 22 insertions(+), 7 deletions(-)