Add a needed break in FxAbs::Resolve()

- Fixed: FxAbs could not resolve floating point constants because it fell
  through to the error case.

src/thingdef/thingdef_expression.cpp

@@ -1576,6 +1576,7 @@ FxExpression *FxAbs::Resolve(FCompileContext &ctx)
 
 		case VAL_Float:
 			value.Float = fabs(value.Float);
+			break;
 
 		default:
 			// shouldn't happen