Fix bug of a + (b - a) == 0 for small positive b and huge a (e.g., 1e18)

I am unable to make a MSAN build to work locally. But I think that this
should fix the problem by looking at the gradient of the fuzzer:

Z.createRadialGradient(1.94159387872, 138.616510533, 2.22085316544e+18,
		       10.3399911492, 176.149195942, 153.078363987);

Bug: chromium:797184
Change-Id: I16c571a378742c4ef50afe9c5baba791da7fc208
Reviewed-on: https://skia-review.googlesource.com/89541
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed