Core/Misc: Fix some static analysis issues

Fix uninitialized values, most of which are false positives, always initialized before being accessed.
Add some asserts and additional NULL checks as sanity checks. Use SpellMgr::EnsureSpellInfo() if the spell id is valid and always supposed to return a valid not-NULL SpellInfo* .
Core/Battlegrounds: Fix log errors about creatures not found

Move "Battleground::GetBGCreature: creature not found for BG" error log to INFO level when the caller expects no creatures to be found, like on Battleground startup.
Closes #3538
Core/Dependencies: Upgrade to jemalloc-3.5.1
DB/Text: Added the missing page texts for the book "Portals are NOT Garbage Bins!"

Closes #3362
Merge pull request #1 from TrinityCore/master

pull