Avoid infinite loop in display of invisible text in strings

Avoid infinite loop in display of invisible text in strings

* src/xdisp.c (handle_invisible_prop): If the next change of
invisibility spec does not mean the beginning of a visible text,
update the string position from which to start the search for the
next invisibility change.  This avoids an infinite loop when we
have more than one invisibility spec that are made inactive by
buffer-invisibility-spec.  Simplify code.  (Bug#21200)

* test/redisplay-testsuite.el (test-redisplay-4): Add a test case
for the situation that caused bug #21200.