fix SIGSEGV in VM in opcLdDeref

[timotheecour]

• fix regression: SIGSEGV with addr + template in static context timotheecour/Nim#88
• refactored code in common between opcWrDeref and opcLdDeref
• should be backported to 1.2
• see tests/vm/tref.nim

block:
  # fix https://github.com/timotheecour/Nim/issues/88
  template fun() =
    var s = @[10,11,12]
    var a = s[0].addr
    a[] += 100 # was giving SIGSEGV
    doAssert a[] == 110
  static: fun()

/cc @Araq failure unrelated nitely/nim-regex#60 EDIT: fixed now