fmt: Add fmt_strnlen function

sys/fmt/fmt.c

@@ -83,6 +83,15 @@ size_t fmt_strlen(const char *str)
     return (tmp - str);
 }
 
+size_t fmt_strnlen(const char *str, size_t maxlen)
+{
+    const char *tmp = str;
+    while(*tmp && maxlen--) {
+        tmp++;
+    }
+    return (tmp - str);
+}
+
 size_t fmt_str(char *out, const char *str)
 {
     int len = 0;

sys/include/fmt.h

@@ -294,6 +294,17 @@ size_t fmt_float(char *out, float f, unsigned precision);
  */
 size_t fmt_strlen(const char *str);
 
+/**
+ * @brief Count at most @p maxlen characters until '\0' (exclusive) in @p str
+ *
+ * @param[in]   str     Pointer to string
+ * @param[in]   maxlen  Maximum number of chars to count
+ *
+ * @return      nr of characters in string @p str points to, or @p maxlen if no
+ *              null terminator is found within @p maxlen chars
+ */
+size_t fmt_strnlen(const char *str, size_t maxlen);
+
 /**
  * @brief Copy null-terminated string (excluding terminating \0)
  *