[BUGFIX] Multi-table relationship not recognized in t3lib_loadDBGroup

If a multi-table relationship has $MM_oppositeFieldConf['allowed']=='*',
then it is not recognized as such one in t3lib_loadDBGroup::start().

Change-Id: Ifbf8a71af8a9d0a0636d8a2332999c402a717559
Resolves: #34148
Releases: 4.7, 4.6, 4.5
Reviewed-on: https://review.typo3.org/9150
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Mario Rimann
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters