Decode floats with overflowing mantissa.

This is a follow up to #246 . Floats with overflowing mantissa needs to be rounded the proper way.

Relevant code:

Lines 17 to 19 in aa64387

    
           if (overflow) { 
        
           	return getError(location, "Integral too large to fit in 64 bits."); 
        
           }

	if (overflow) {
	return getError(location, "Integral too large to fit in 64 bits.");
	}