Assertion fix for ProblemModel

Summary:
The problem was caused by cases when some problem has relative paths in it's final location.
For example, Valgrind DRD tool can produce urls like this:

/build/glibc-t3gR2i/glibc-2.23/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S

If we try to open such problems then assertion inside IndexedString constructor happens (serialization/indexedstring.cpp:256):

Q_ASSERT(url == url.adjusted(QUrl::NormalizePathSegments));

Test Plan: Tested on master branch

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4641