get_value_string: Null terminate after the call to snprintf

It is impossible that the snprintf will fill the entire namebuf
but just follow the idiom to make sure it is null terminated.

Related: Coverity CID 1040676

src/utils.c

@@ -31,6 +31,7 @@ const char *get_value_string(const struct value_string *vs, uint32_t val)
 	}
 
 	snprintf(namebuf, sizeof(namebuf), "unknown 0x%x", val);
+	namebuf[sizeof(namebuf) - 1] = '\0';
 	return namebuf;
 }