Float constants end with "f"; assign float constants to gfloats.

This squelches some compiler warnings about assigning double values to
floats (no, there's no precision in 0 to lose, but the compiler warns
anyway).

Change-Id: Ia49618cb8582c5afcdc5bdc5334fe20724b85eda
Reviewed-on: https://code.wireshark.org/review/3762
Reviewed-by: Guy Harris <guy@alum.mit.edu>