Fix memory leak on getPidTagSubject

Valgrind crafted report:

=40967== 128,927 (53,750 direct, 75,177 indirect) bytes in 383 blocks are definitely lost in loss record 10,104 of 10,179
==40967==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==40967==    by 0x86EDE3D: talloc_strndup (talloc.c:613)
==40967==    by 0x34D2BB9E: _i_NSString_MAPIStoreDataTypes_asUnicodeInMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34D2025B: _i_MAPIStoreMailMessage__getPidTagNormalizedSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34CFCD40: _i_MAPIStoreObject__getProperty_withTag_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34CFBE21: _i_MAPIStoreMessage__getPidTagSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34CFBEB6: _i_MAPIStoreMessage__getPidTagOriginalSubject_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34CFCD40: _i_MAPIStoreObject__getProperty_withTag_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
==40967==    by 0x34CFD07E: _i_MAPIStoreObject__getProperties_withTags_andCount_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
Remove an attempt to double free content

As content is freed by resetExceptResult message received by reset
which has been called by dealloc as well
Attempt to sync an UID in setChangeKey to avoid crashing

This may happen if between sync cache and setChangeKey a modSeq
is making synchroniseCache not retrieve the newly stored message.

This should fix the following crash:

https://tracker.zentyal.org/issues/2673
Merge pull request #61 from Zentyal/ejhernandez/memory-leaks

Fix memory leak on getPidTagSubject
abort if attemp to sync an UID in setChangekey fails
Merge pull request #62 from Zentyal/ejhernandez/fix-space-temporal-crash-setChangeKey

Attempt to sync an UID in setChangeKey to avoid crashing
Merge pull request #63 from Zentyal/master

Update trusty