Fix infinite loop when walking up the directory hierarchy (#1183)

A single find_package(harfbuzz) line in user code resulted in this loop
getting stuck when _harfbuzz_libdir_iter became "/".

src/harfbuzz-config.cmake.in

@@ -12,7 +12,11 @@ set(_harfbuzz_libdir "@libdir@")
 string(REPLACE "${_harfbuzz_remove_string}/" "" _harfbuzz_libdir "${_harfbuzz_libdir}")
 set(_harfbuzz_libdir_iter "${_harfbuzz_libdir}")
 while (_harfbuzz_libdir_iter)
+  set(_harfbuzz_libdir_prev_iter "${_harfbuzz_libdir_iter}")
   get_filename_component(_harfbuzz_libdir_iter "${_harfbuzz_libdir_iter}" DIRECTORY)
+  if (_harfbuzz_libdir_prev_iter STREQUAL _harfbuzz_libdir_iter)
+    break()
+  endif ()
   get_filename_component(_harfbuzz_prefix "${_harfbuzz_prefix}" DIRECTORY)
 endwhile ()
 unset(_harfbuzz_libdir_iter)