[BUGFIX] Remove additional slashes from typolink urls if prefixing

[BUGFIX] Remove additional slashes from typolink urls if prefixing

Typolink generates absolute uris with a leading slash. When prefixing
this with the base uri, we have to make sure, there is just one
slash in result. To prevent these situations we also trigger the
UriBuilder itself to generate an absolute path.

Thanks to Chetan Thapliyal.

Change-Id: Iae8755514fbdfc4eb27a5a6eab620e55144d912e
Fixes: #33788
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/8860
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Daniel Lorenz
Tested-by: Daniel Lorenz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn