add some asserts for mipmaps, and cleanup fFilterLevel if mipping failed

BUG=skia:
R=scroggo@google.com, humper@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/240633003

git-svn-id: http://skia.googlecode.com/svn/trunk@14241 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed