Fix out-of-bounds array access in preposition vocabulary

[fuulish]

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[fuulish]

Hi, @devshane, I opened this pull request independently of PR #16, because I was transferring it from a separate, but similar implementation (richelbilderbeek/Zork#2). It'd be nice if either PR could be merged, because they fix the undefined behavior incurred by the array-out-of-bounds access (prplnt <-> pvoc).

[Jan200101]

merging this over #16

amended the commit to remove the volatile from the locals which was the previous workaround when I believed this to be caused by a compiler optimization error, this error originated from the original Fortran version and only became an issue when any agressive optimizations are used.