fixes for VecPlaceArray/VecResetArray

[Lisandro Dalcin]

I've pushed some fixes for VecPlaceArray/VecResetArray, trying to the
pair a bit more smarter. However, the I'm not sure what's the use case
in VecPlaceArray for accepting NULL for the array. Just in case, I did
not changed this, but if NULL is actually pased, then the call is now
non-op. Is this fine? I'm missing something??

-- 
Lisandro Dalcín
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Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
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