modify the value pointed by NumberOfPages to accordance with IN&OUT modifier's syntax.
[Impaction]
add comments for NumberOfPages parameter and assign a correct value to it.
[Reference Info]
NumberOfPages is only updated inside the if statement on line 725. If buffer is big enough the actual number of pages occupied by the image is not returned
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@5112 6f19259b-4bc3-4df7-8a09-765794883524
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