Fix memory leak on getPidTagSubject

An example from Valgrind report (there are hundreds with different traces):
 
    128,927 (53,750 direct, 75,177 indirect) bytes in 383 blocks are definitely lost in loss record 10,104 of 10,179
     at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
     by 0x86EDE3D: talloc_strndup (talloc.c:613)
     by 0x34D2BB9E: _i_NSString_MAPIStoreDataTypes_asUnicodeInMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34D2025B: _i_MAPIStoreMailMessage__getPidTagNormalizedSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34CFCD40: _i_MAPIStoreObject__getProperty_withTag_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34CFBE21: _i_MAPIStoreMessage__getPidTagSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34CFBEB6: _i_MAPIStoreMessage__getPidTagOriginalSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34CFCD40: _i_MAPIStoreObject__getProperty_withTag_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
     by 0x34CFD07E: _i_MAPIStoreObject__getProperties_withTags_andCount_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)