*: s/"/bin/sh"/DEFAULT_SHELL, run_shell() API fix, remove unneeded strdup

function                                             old     new   delta
run_shell                                            157     166      +9
su_main                                              477     470      -7
sulogin_main                                         515     503     -12

Signed-off-by: Ladislav Michl <Ladislav.Michl@seznam.cz>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/run_shell.c b/libbb/run_shell.c
index 4608a24..4d92c3c 100644
--- a/libbb/run_shell.c
+++ b/libbb/run_shell.c
@@ -49,15 +49,14 @@
 
 #endif
 
-/* Run SHELL, or DEFAULT_SHELL if SHELL is empty.
-   If COMMAND is nonzero, pass it to the shell with the -c option.
-   If ADDITIONAL_ARGS is nonzero, pass it to the shell as more
-   arguments.  */
-
+/* Run SHELL, or DEFAULT_SHELL if SHELL is "" or NULL.
+ * If COMMAND is nonzero, pass it to the shell with the -c option.
+ * If ADDITIONAL_ARGS is nonzero, pass it to the shell as more
+ * arguments.  */
 void FAST_FUNC run_shell(const char *shell, int loginshell, const char *command, const char **additional_args)
 {
 	const char **args;
-	int argno = 1;
+	int argno;
 	int additional_args_cnt = 0;
 
 	for (args = additional_args; args && *args; args++)
@@ -65,11 +64,13 @@
 
 	args = xmalloc(sizeof(char*) * (4 + additional_args_cnt));
 
-	args[0] = bb_get_last_path_component_nostrip(xstrdup(shell));
+	if (!shell || !shell[0])
+		shell = DEFAULT_SHELL;
 
+	args[0] = bb_get_last_path_component_nostrip(shell);
 	if (loginshell)
 		args[0] = xasprintf("-%s", args[0]);
-
+	argno = 1;
 	if (command) {
 		args[argno++] = "-c";
 		args[argno++] = command;
@@ -79,6 +80,7 @@
 			args[argno++] = *additional_args;
 	}
 	args[argno] = NULL;
+
 #if ENABLE_SELINUX
 	if (current_sid)
 		setexeccon(current_sid);