Runtime: Add an early exit condition to vfs.dlopen

This makes it possible to chain the function with ffi.load, so that the same code will work both during local development and when bundled as a self-contained LUAZIP application.

Runtime/Libraries/vfs.lua

@@ -24,6 +24,9 @@ local vfs = {
 		#pragma pack(pop)
 	]],
 	cachedAppBundles = {},
+	errorStrings = {
+		MISSING_APP_BUNDLE = "Not a LUAZIP app bundle",
+	},
 }
 
 function vfs.decode(fileContents)
@@ -105,6 +108,11 @@ function vfs.searcher(zipApp, moduleName)
 end
 
 function vfs.dlopen(zipApp, libraryName)
+	if not zipApp then
+		-- Ensure it's a NOOP if run from a regular script file that may also be bundled
+		return nil, vfs.errorStrings.MISSING_APP_BUNDLE
+	end
+
 	validation.validateString(libraryName, "libraryName")
 
 	libraryName = vfs.dlname(libraryName)

Tests/BDD/vfs-library.spec.lua

@@ -134,6 +134,12 @@ describe("vfs", function()
 	end)
 
 	describe("dlopen", function()
+		it("should fail if no app bundle was provided", function()
+			assertFailure(function()
+				return vfs.dlopen(nil, "foo")
+			end, vfs.errorStrings.MISSING_APP_BUNDLE)
+		end)
+
 		it("should throw if an invalid library name was passed", function()
 			assertThrows(function()
 				vfs.dlopen({}, nil)