[MSAN] Fix wordexp interception when WRDE_DOOFFS is used

Handle the case of wordexp being invoked with WRDE_DOOFFS and
we.we_offs set to a positive value, which will result in NULL
entries prepended to the result. With this change the entire
result, containing both NULL and actual entries, is unpoisoned.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D108646