Fix lengths of interface names

Use helper function similar to copy correctly limited names into
buffers.

src/dnsmasq.h

@@ -1252,6 +1252,7 @@ int legal_hostname(char *name);
 char *canonicalise(char *in, int *nomem);
 unsigned char *do_rfc1035_name(unsigned char *p, char *sval, char *limit);
 void *safe_malloc(size_t size);
+void safe_strncpy(char *dest, const char *src, size_t size);
 void safe_pipe(int *fd, int read_noblock);
 void *whine_malloc(size_t size);
 int sa_len(union mysockaddr *addr);