Fix NaN handling

Author: kgryte

lib/node_modules/@stdlib/stats/incr/mstdev/lib/main.js

@@ -112,7 +112,7 @@ function incrmstdev( W, mean ) {
 				return null;
 			}
 			if ( N === 1 ) {
-				return 0.0;
+				return ( isnan( M2 ) ) ? NaN : 0.0;
 			}
 			if ( N < W ) {
 				return sqrt( M2 / (N-1) );

lib/node_modules/@stdlib/stats/incr/mstdev/test/test.js

@@ -355,8 +355,10 @@ tape( 'if provided `NaN`, the accumulated value is `NaN` for at least `W` invoca
 		v = acc( data[ i ] );
 		if ( isnan( expected[ i ] ) ) {
 			t.equal( isnan( v ), true, 'returns expected value for window '+i );
+			t.equal( isnan( acc() ), true, 'returns expected value for window '+i );
 		} else {
 			t.equal( v, expected[ i ], 'returns expected value for window '+i );
+			t.equal( acc(), expected[ i ], 'returns expected value for window '+i );
 		}
 	}
 	t.end();
@@ -417,8 +419,10 @@ tape( 'if provided `NaN`, the accumulated value is `NaN` for at least `W` invoca
 		v = acc( data[ i ] );
 		if ( isnan( expected[ i ] ) ) {
 			t.equal( isnan( v ), true, 'returns expected value for window '+i );
+			t.equal( isnan( acc() ), true, 'returns expected value for window '+i );
 		} else {
 			t.equal( v, expected[ i ], 'returns expected value for window '+i );
+			t.equal( acc(), expected[ i ], 'returns expected value for window '+i );
 		}
 	}
 	t.end();