msi: Passing NULL for szBuffer and NULL for pcchValueBuf into MsiGetProductInfo{A, W} shouldn't crash.

[Robert Shearman]

Add a test for this.
---
  dlls/msi/msi.c       |   27 ++++++++++++++++-----------
  dlls/msi/tests/msi.c |    4 ++++
  2 files changed, 20 insertions(+), 11 deletions(-)
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