GCS_MAVLink: fixed overrun of text passed to other libraries

this caused corruption in log files, and was passing a
non-nul-terminated string to notify and other libraries

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -1836,7 +1836,7 @@ void GCS::send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list, u
         // send_text can be called from multiple threads; we must
         // protect the "text" member with _statustext_sem
         hal.util->vsnprintf(statustext_printf_buffer, sizeof(statustext_printf_buffer), fmt, arg_list);
-        memcpy(first_piece_of_text, statustext_printf_buffer, ARRAY_SIZE(first_piece_of_text));
+        memcpy(first_piece_of_text, statustext_printf_buffer, ARRAY_SIZE(first_piece_of_text)-1);
 
         // filter destination ports to only allow active ports.
         statustext_t statustext{};