Update exim_lock.c Enhance Security with strncpy

[Shivam7-1]

In this PR replaces instances of strcpy() with strncpy() to mitigate the risk of buffer overflows. strncpy() ensures safer handling of strings by limiting the number of copied characters and adding null-termination, enhancing the overall security and stability of the code.

[Cyborgscode]

A quick check with the man page of strncpy ... it has 3 arguments :

char *strncpy(char dst[restrict .dsize], const char *restrict src, size_t dsize);

I'm wondering what the none-optional third one does ...

[Cyborgscode]

Just for education:

strncpy(), even if used correctly, which you didn't, would not enhance security, because the buffersize check is right infront of that strcpy() call :

  if ((int)strlen(pw->pw_dir) + (int)strlen(filename) + 1 > sizeof(buffer))
    {
    printf("exim_lock: expanded file name %s%s is too long", pw->pw_dir,
      filename);
    exit(EXIT_FAILURE);
    }

  strcpy(buffer, pw->pw_dir);
  strcat(buffer, filename);
  filename = buffer;
  }