patch 8.1.2061: MS-Windows GUI: ":sh" crashes when trying to use a terminal

Problem:    MS-Windows GUI: ":sh" crashes when trying to use a terminal.
Solution:   Check for a NULL command. (Yasuhiro Matsumoto, closes #4958)

Author: brammool

src/os_win32.c

@@ -4657,12 +4657,14 @@ mch_call_shell(
     {
 	char_u	*cmdbase = cmd;
 
-	// Skip a leading quote and (.
-	while (*cmdbase == '"' || *cmdbase == '(')
-	    ++cmdbase;
+	if (cmdbase != NULL)
+	    // Skip a leading quote and (.
+	    while (*cmdbase == '"' || *cmdbase == '(')
+		++cmdbase;
 
 	// Check the command does not begin with "start "
-	if (STRNICMP(cmdbase, "start", 5) != 0 || !VIM_ISWHITE(cmdbase[5]))
+	if (cmdbase == NULL
+		|| STRNICMP(cmdbase, "start", 5) != 0 || !VIM_ISWHITE(cmdbase[5]))
 	{
 	    // Use a terminal window to run the command in.
 	    x = mch_call_shell_terminal(cmd, options);

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2061,
 /**/
     2060,
 /**/