cmd: Set errorlevel to 0 when 'call' is invoked with an empty string.

programs/cmd/tests/test_builtins.cmd

@@ -3149,6 +3149,9 @@ echo %ErrorLevel% should be 7
 if errorlevel 7 echo setting var worked too well, bad
 call :setError 3
 echo %ErrorLevel% should still be 7
+rem Verify that (call ) sets errorlevel to 0
+(call )
+if errorlevel 1 echo errorlevel should have been 0
 
 echo ------------ Testing GOTO ------------
 if a==a goto dest1

programs/cmd/wcmdmain.c

@@ -1053,6 +1053,11 @@ void WCMD_run_program (WCHAR *command, BOOL called)
   firstParam = WCMD_parameter(command, 0, NULL, FALSE, TRUE);
   if (!firstParam) return;
 
+  if (!firstParam[0]) {
+      errorlevel = 0;
+      return;
+  }
+
   /* Calculate the search path and stem to search for */
   if (wcspbrk (firstParam, delims) == NULL) {  /* No explicit path given, search path */
     static const WCHAR curDir[] = {'.',';','\0'};