style: avoid strncpy

syntax-check seems to dislike strncpy.  The GNU Coreutils replaced
their uses of strncpy with stpncpy.

strlcpy is not an option.
  http://sources.redhat.com/ml/libc-alpha/2002-01/msg00159.html
  http://sources.redhat.com/ml/libc-alpha/2002-01/msg00011.html
  http://lists.gnu.org/archive/html/bug-gnulib/2004-09/msg00181.html

* src/glyphs.c: Use stpncpy.

lib/.gitignore

@@ -313,6 +313,7 @@
 /stdlib.h
 /stdlib.in.h
 /stpcpy.c
+/stpncpy.c
 /strchrnul.c
 /strchrnul.valgrind
 /strdup.c

m4/.gitignore

@@ -175,6 +175,7 @@
 /stdio_h.m4
 /stdlib_h.m4
 /stpcpy.m4
+/stpncpy.m4
 /strchrnul.m4
 /strdup.m4
 /strerror.m4

src/glyphs.c

@@ -63,8 +63,8 @@ static long
 on_success (const char *buf, size_t buflen, void *callback_arg)
 {
   callback_arg_t *arg = (callback_arg_t *) callback_arg;
-  assert (buflen < sizeof arg->buf);
-  strncpy (arg->buf, buf, buflen);
+  assert (buflen + 1 < sizeof arg->buf);
+  *stpncpy (arg->buf, buf, buflen) = '\0';
   return 1;
 }
 
@@ -73,7 +73,7 @@ on_failure (unsigned code MAYBE_UNUSED, const char *msg MAYBE_UNUSED,
             void *callback_arg)
 {
   callback_arg_t *arg = (callback_arg_t *) callback_arg;
-  assert (strlen (arg->fallback) < sizeof arg->buf);
+  assert (strlen (arg->fallback) + 1 < sizeof arg->buf);
   strcpy (arg->buf, arg->fallback);
   return 0;
 }