Fix #74934.
* expression.cs (BinaryResolveOperator): If one of the operands of
an equality comparison is 'null' and the other is a pointer type,
convert the null to a NullPointer.
* convert.cs (ImplicitReferenceConversion): If the expression is a
NullLiteral and the target type is a pointer type, return a
NullPointer instead.
(ImplicitConversionStandard): Likewise.
In tests:
* test-380.cs: New test from #74934.
svn path=/trunk/mcs/; revision=44495
