Fix a bug in single->half conversion.

The original C code had if(m & 0x00800000) which is true if the
expresssion does not evaluate to zero. This was inncorretly translated
to the C# code if((m & 0x00800000) == 1) which only evaluates true if
the expression evalaute to 1, which it never does. The correct test is
to test not equal to zero (!= 0).
Merge pull request #128 from Frassle/half-fix

Fix a bug in single->half conversion.