PVS-Studio warning V557 for Firefox

class nsBaseStatis : public nsStatis {
public:
  ...
  PRUint32 mLWordLen[10];
  ...
  nsBaseStatis::nsBaseStatis(
    unsigned char aL, unsigned char aH, float aR)
  {
    ...
    for(PRUint32 i = 0; i < 20; i++)
       mLWordLen[i] = 0;
    ...
  }
}

  ...
};

This is what should have been written here: for(PRUint32 i = 0; i < 10; i++) or: for(PRUint32 i = 0; i < sizeof(mLWordLen)/sizeof(mLWordLen[0]); i++)

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V557 Array overrun is possible. The value of 'i' index could reach 19. detectcharset.cpp 89

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/