Correctly unlock the previously locked value in addmdocs() (refs #3883)

A lock was not properly disposed, thus preventing adding more than one element
in a multiple attribute.
Corrected duplication of array's rows containing multiples docids (refs #3883)
Merge pull request #800 from eguaj/3.2-A3883

Corrected duplication of array's rows containing multiples docids (closes #3883)