fix check on number of arguments

[VinInn]

fix trivial bug that potentially make all builds to get stuck..

[cmsbuild]

A new Pull Request was created by @VinInn (Vincenzo Innocente) for CMSSW_7_1_X.

fix check on number of arguments

It involves the following packages:

RecoPixelVertexing/PixelTriplets

@nclopezo, @cmsbuild, @anton-a, @thspeer, @slava77, @Degano can you please review it and eventually sign? Thanks.
@cerati, @GiacomoSguazzoni, @rovere this is something you requested to watch as well.
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@nclopezo, @ktf you are the release manager for this.
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[ktf]

Clearly currently wrong. Approving. Complain if not ok.