refactor: add missing assertion

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---

Author: kgryte

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

@@ -108,6 +108,9 @@ function assign( x, y, options, predicate, thisArg ) {
 	// Case: assign( x, y, predicate )
 	if ( nargs < 4 ) {
 		cb = options;
+		if ( !isFunction( cb ) ) {
+			throw new TypeError( format( 'invalid argument. Third argument must be a function. Value: `%s`.', cb ) );
+		}
 	}
 	// Case: assign( x, y, options, predicate, thisArg )
 	else if ( nargs > 4 ) {
@@ -141,6 +144,7 @@ function assign( x, y, options, predicate, thisArg ) {
 			throw err;
 		}
 	}
+	// When a list of dimensions is not provided, reduce the entire input array across all dimensions...
 	if ( opts.dims === null ) {
 		opts.dims = zeroTo( N );
 	}