glib-aux: add nm_str_truncate() helper

src/libnm-glib-aux/nm-macros-internal.h

@@ -495,6 +495,17 @@ nm_strdup_not_empty(const char *str)
     return !nm_str_is_empty(str) ? g_strdup(str) : NULL;
 }
 
+static inline char *
+nm_str_truncate(char *str)
+{
+    /* This is trivial, and is only useful in a macro, to
+     * ensure that we access the macro argument only once. */
+    nm_assert(str);
+
+    str[0] = '\0';
+    return str;
+}
+
 static inline char *
 nm_str_realloc(char *str)
 {