Print a warning in CHANcreate, not returning NULL

Let's play safe, and not return NULL when the file descriptor is too
large.  The rest of the code currently does not handle such a response,
but ensures the file descriptor is valid before calling CHANcreate().

see #273

innd/chan.c

@@ -186,11 +186,16 @@ CHANcreate(int fd, enum channel_type type, enum channel_state state,
     struct buffer out = {0, 0, 0, NULL};
 
     /* Last time to ensure the file descriptor is not too high.  Callers have
-       normally already handled that case.
-       Note that the channel table, created in CHANsetup, is known to have
+       normally already handled that case before and not called CHANcreate().
+       Note that the channel table, created in CHANsetup(), is known to have
        enough room. */
-    if (!isvalidfd(fd))
-        return NULL;
+    if (!isvalidfd(fd)) {
+        /* Go on without returning NULL, in case a caller is not prepared to
+           receive such a response. */
+        syswarn("%s file descriptor %d too high in CHANcreate (see "
+                "rlimitnofile in inn.conf)",
+                LogName, fd);
+    }
 
     /* Currently, we do no dynamic allocation, but instead assume that the
        channel table is sized large enough to hold all possible file