xz: Refactor to remove is_empty_filename().

Long ago it was used in list.c too but nowadays it's needed
only in io_open_src() so it's nicer to avoid a separate function.

src/xz/file_io.c

@@ -748,8 +748,10 @@ error:
 extern file_pair *
 io_open_src(const char *src_name)
 {
-	if (is_empty_filename(src_name))
+	if (src_name[0] == '\0') {
+		message_error(_("Empty filename, skipping"));
 		return NULL;
+	}
 
 	// Since we have only one file open at a time, we can use
 	// a statically allocated structure.

src/xz/util.c

@@ -260,18 +260,6 @@ my_snprintf(char **pos, size_t *left, const char *fmt, ...)
 }
 
 
-extern bool
-is_empty_filename(const char *filename)
-{
-	if (filename[0] == '\0') {
-		message_error(_("Empty filename, skipping"));
-		return true;
-	}
-
-	return false;
-}
-
-
 extern bool
 is_tty_stdin(void)
 {

src/xz/util.h

@@ -105,10 +105,6 @@ extern void my_snprintf(char **pos, size_t *left, const char *fmt, ...)
 		lzma_attribute((__format__(__printf__, 3, 4)));
 
 
-/// \brief      Check if filename is empty and print an error message
-extern bool is_empty_filename(const char *filename);
-
-
 /// \brief      Test if stdin is a terminal
 ///
 /// If stdin is a terminal, an error message is printed and exit status set