MAINT: fix two typos in test_c99complex.c
ENH: Be more careful with large real parts in npy_cexp

This change is adapted from k_exp.c in FreeBSD.

The idea is to calculate a scaled exponential, which is then multiplied by
cos(imag) and sin(imag) and rescaled to the appropriate value.  This means
that for float64, things like np.exp(complex(729, 1e-16)) which used to give
inf+infj now gives (inf+3.987285262042597e+300j)