Fix profile_count::apply_probability

profile_count::apply_probability misses check for uninitialized probability which leads
to completely random results on applying uninitialized probability to initialized scale.
This can make difference when i.e. inlining -fno-guess-branch-probability function
to -fguess-branch-probability one.

gcc/ChangeLog:

	* profile-count.h (profile_count::apply_probability): Fix
	handling of uninitialized probabilities, optimize scaling
	by probability 1.