Introduce sstrndup wrapper.

include/libi3.h

@@ -127,6 +127,13 @@ void *srealloc(void *ptr, size_t size);
  */
 char *sstrdup(const char *str);
 
+/**
+ * Safe-wrapper around strndup which exits if strndup returns NULL (meaning that
+ * there is no more memory available)
+ *
+ */
+char *sstrndup(const char *str, size_t size);
+
 /**
  * Safe-wrapper around asprintf which exits if it returns -1 (meaning that
  * there is no more memory available)