string-util: add strlen_ptr() helper

strlen_ptr() is to strlen() what streq_ptr() is to streq(): i.e. it
handles NULL strings in a smart way.

src/basic/string-util.h

@@ -200,3 +200,10 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(char *, string_free_erase);
 #define _cleanup_string_free_erase_ _cleanup_(string_free_erasep)
 
 bool string_is_safe(const char *p) _pure_;
+
+static inline size_t strlen_ptr(const char *s) {
+        if (!s)
+                return 0;
+
+        return strlen(s);
+}