net: tcp: Fix possible buffer underflow

Fix possible underflow in tcp flags parse.

Signed-off-by: Flavio Ceolin <[email protected]>
(cherry picked from commit ac2e13b)

Author: Flavio Ceolin

subsys/net/ip/tcp2.c

@@ -231,7 +231,9 @@ static const char *tcp_flags(uint8_t flags)
 			len += snprintk(buf + len, BUF_SIZE - len, "URG,");
 		}
 
-		buf[len - 1] = '\0'; /* delete the last comma */
+		if (len > 0) {
+			buf[len - 1] = '\0'; /* delete the last comma */
+		}
 	}
 #undef BUF_SIZE
 	return buf;