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/hexdecoct.c

@@ -25,6 +25,7 @@
 #include "alloc-util.h"
 #include "hexdecoct.h"
 #include "macro.h"
+#include "string-util.h"
 #include "util.h"
 
 char octchar(int x) {
@@ -569,7 +570,7 @@ static int base64_append_width(char **prefix, int plen,
 
         lines = (len + width - 1) / width;
 
-        slen = sep ? strlen(sep) : 0;
+        slen = strlen_ptr(sep);
         t = realloc(*prefix, plen + 1 + slen + (indent + width + 1) * lines);
         if (!t)
                 return -ENOMEM;