osmo-sim-test: Fix read_binary() return check

if we expect rmsg to be set, we should check for it...
diff --git a/utils/osmo-sim-test.c b/utils/osmo-sim-test.c
index 244c2df..9d1b481 100644
--- a/utils/osmo-sim-test.c
+++ b/utils/osmo-sim-test.c
@@ -350,7 +350,7 @@
 			uint16_t remain_len = i - offset;
 			uint16_t read_len = OSMO_MIN(remain_len, 256);
 			rmsg = read_binary(chan, offset, read_len);
-			if (!msg)
+			if (!rmsg)
 				return -EIO;
 			offset += read_len;
 			printf("Content: %s\n",