Avoid false static analyzer warning on impossible snprintf() error path

Explicitly check that snprintf() return value is not negative to silence
a static analyzer. This call cannot fail, but it is simpler to do this
than try to figure out other ways of getting rid of false warnings.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
diff --git a/atheros.c b/atheros.c
index 6367427..6b1eadc 100644
--- a/atheros.c
+++ b/atheros.c
@@ -44,6 +44,8 @@
 		return -2;
 
 	i = snprintf(resp, sizeof(resp), "resp,");
+	if (i < 0)
+		return -2;
 	pos = buf;
 	while (*pos && i + 1 < (int) sizeof(resp)) {
 		char c = *pos++;