fix: apply suggestions from code review

Author: headlessNode

lib/node_modules/@stdlib/ndarray/base/diagonal/lib/main.js

@@ -114,7 +114,10 @@ function diagonal( x, dims, k, writable ) {
 		L = 0;
 	}
 	// Adjust the offset so that we point to the first element along the specified diagonal:
-	offset = getOffset( x ) + ( co*sc ) + ( ro*sr );
+	offset = getOffset( x );
+	if ( L > 0 ) {
+		offset += ( co*sc ) + ( ro*sr );
+	}
 
 	// Drop the specified dimensions and append the diagonal dimension:
 	shape = [];