don't claim to be an erf file if it isn't (e.g. if the file_read call fails)

I don't know if this is the right way to fix it, but it seems at least better than before

svn path=/trunk/; revision=16971

wiretap/erf.c

@@ -127,7 +127,7 @@ int erf_open(wtap *wth, int *err, gchar **err_info _U_)
 			if ((*err = file_error(wth->fh)) != 0)
 				return -1;
 			else
-				break; /* eof */
+		        return 0;
 		}
 
 		packet_size = g_ntohs(header.rlen) - sizeof(header);