uri_get(NULL) and uri_put(NULL) are explicitly allowed.

src/core/cwuri.c

 /**
  * Change working URI
  *
- * @v uri		New working URI
+ * @v uri		New working URI, or NULL
  */
 void churi ( struct uri *uri ) {
-	if ( cwuri )
-		uri_put ( cwuri );
+	uri_put ( cwuri );
 	cwuri = uri_get ( uri );
 }

src/include/gpxe/uri.h

 /**
  * Increment URI reference count
  *
- * @v uri		URI
- * @ret uri		URI
+ * @v uri		URI, or NULL
+ * @ret uri		URI as passed in
  */
 static inline __attribute__ (( always_inline )) struct uri *
 uri_get ( struct uri *uri ) {