Fix for corner cases in code handling leading "* " decorations in block comments

Summary:
Fixes problems that lead to incorrect formatting of these and similar snippets:
/*
 **
 */

/*
 **/

/*
 * */

/*
 *test
 */

Clang-format used to think that all the cases above use "* " decoration, and
failed to calculate insertion position properly. It also used to remove leading
"* " in the last line.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1113