refactor: add missing assertion

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---

Author: kgryte

lib/node_modules/@stdlib/ndarray/count-if/lib/main.js

@@ -113,6 +113,9 @@ function countIf( x, options, predicate, thisArg ) {
 	// Case: countIf( x, predicate )
 	if ( nargs < 3 ) {
 		cb = options;
+		if ( !isFunction( cb ) ) {
+			throw new TypeError( format( 'invalid argument. Second argument must be a function. Value: `%s`.', cb ) );
+		}
 	}
 	// Case: countIf( x, options, predicate, thisArg )
 	else if ( nargs > 3 ) {