chan_dahdi.c: Resolve a format-truncation build warning.

With gcc (Ubuntu 11.2.0-19ubuntu1) 11.2.0:

> chan_dahdi.c:4129:18: error: ‘%s’ directive output may be truncated
>   writing up to 255 bytes into a region of size between 242 and 252
>   [-Werror=format-truncation=]

This removes the error-prone sizeof(...) calculations in favor of just
doubling the size of the base buffer.

Change-Id: I2d276785286730d3d5d0a921bcea2e065dbf27c5

channels/chan_dahdi.c

@@ -3943,7 +3943,7 @@ static void dahdi_r2_on_context_log(openr2_context_t *r2context, openr2_log_leve
 {
 #define CONTEXT_TAG "Context - "
 	char logmsg[256];
-	char completemsg[sizeof(logmsg) + sizeof(CONTEXT_TAG) - 1];
+	char completemsg[sizeof(logmsg) * 2];
 	vsnprintf(logmsg, sizeof(logmsg), fmt, ap);
 	snprintf(completemsg, sizeof(completemsg), CONTEXT_TAG "%s", logmsg);
 	dahdi_r2_write_log(level, completemsg);
@@ -3956,10 +3956,11 @@ static void dahdi_r2_on_chan_log(openr2_chan_t *r2chan, openr2_log_level_t level
 {
 #define CHAN_TAG "Chan "
 	char logmsg[256];
-	char completemsg[sizeof(logmsg) + sizeof(CHAN_TAG) - 1];
+	char completemsg[sizeof(logmsg) * 2];
 	vsnprintf(logmsg, sizeof(logmsg), fmt, ap);
 	snprintf(completemsg, sizeof(completemsg), CHAN_TAG "%d - %s", openr2_chan_get_number(r2chan), logmsg);
 	dahdi_r2_write_log(level, completemsg);
+#undef CHAN_TAG
 }
 
 static int dahdi_r2_on_dnis_digit_received(openr2_chan_t *r2chan, char digit)