PVS-Studio warning V557 for WebRTC

cc_string_t lineLabels[MAX_CONFIG_LINES+1] = {0};

void ccsnap_set_line_label(int btn, cc_string_t label) {
  ....
  if ( btn > 0 && btn <= MAX_CONFIG_LINES+1 ) {
    ....
    if ( lineLabels[btn] == NULL ) {
    ....
  }
  ....
}

This suspicious code was found in WebRTC project by PVS-Studio static code analyzer.
Warning message is:
V557 Array overrun is possible. The value of 'btn' index could reach 52. ccapi_snapshot.c 38

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/