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/bus-label.h

@@ -23,9 +23,11 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "string-util.h"
+
 char *bus_label_escape(const char *s);
 char *bus_label_unescape_n(const char *f, size_t l);
 
 static inline char *bus_label_unescape(const char *f) {
-        return bus_label_unescape_n(f, f ? strlen(f) : 0);
+        return bus_label_unescape_n(f, strlen_ptr(f));
 }

src/basic/escape.c

@@ -314,7 +314,7 @@ int cunescape_length_with_prefix(const char *s, size_t length, const char *prefi
 
         /* Undoes C style string escaping, and optionally prefixes it. */
 
-        pl = prefix ? strlen(prefix) : 0;
+        pl = strlen_ptr(prefix);
 
         r = new(char, pl+length+1);
         if (!r)

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;

src/basic/string-util.c

@@ -215,7 +215,7 @@ char *strnappend(const char *s, const char *suffix, size_t b) {
 }
 
 char *strappend(const char *s, const char *suffix) {
-        return strnappend(s, suffix, suffix ? strlen(suffix) : 0);
+        return strnappend(s, suffix, strlen_ptr(suffix));
 }
 
 char *strjoin_real(const char *x, ...) {
@@ -707,7 +707,7 @@ char *strextend(char **x, ...) {
 
         assert(x);
 
-        l = f = *x ? strlen(*x) : 0;
+        l = f = strlen_ptr(*x);
 
         va_start(ap, x);
         for (;;) {

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);
+}