-fsanitize=undefined detects undefined behaviour in signed shift overflow

[Sergei Trofimovich]

Hello ncurses maintainers!

In search for an unrelated bug I built a few local tools with
`-fsanitize=undefined` gcc option to catch an suspected undefined
behaviour.

Among other things ncurses was flagged in ncurses-based applications as:
  ncurses/tinfo/read_entry.c:111:19:
    runtime error: left shift of 255 by 24 places cannot be represented in type 
'int'

which looks like a real (perhaps minor) problem:

  #else
  static size_t
  convert_32bits(char *buf, NCURSES_INT2 *Numbers, int count)
  {
      int i, j;
      unsigned char ch;
      for (i = 0; i < count; i++) {
          int value = 0;
          for (j = 0; j < SIZEOF_32BITS; ++j) {
              ch = UChar(*buf++);
/* line 111: * / value |= (ch << (8 * j));
          }
          if (value == -1)
              Numbers[i] = ABSENT_NUMERIC;
          else if (value == -2)
              Numbers[i] = CANCELLED_NUMERIC;
          else if (value > MAX_OF_TYPE(NCURSES_INT2))
              Numbers[i] = MAX_OF_TYPE(NCURSES_INT2);
          else
              Numbers[i] = (short) value;
          TR(TRACE_DATABASE, ("get Numbers[%d]=%d", i, Numbers[i]));
      }
      return SIZEOF_SHORT;
  }

Would it make sense to clean code up to avoid overflow for signed shift
in line 111?

Steps to reproduce on x86_64-pc-linux-gnu gcc-11.1.0 system:

    $ git clone https://github.com/mirror/ncurses.git
    $ cd ncurses
    $ ./configure --enable-termcap CFLAGS='-O2 -g -fsanitize=undefined' 
LDFLAGS='-O2 -g -fsanitize=undefined'
    $ make
    $ # progs/infocmp
    ../ncurses/./tinfo/read_entry.c:111:19: runtime error: left shift of 255 by 
24 places cannot be represented in type 'int'
    #       Reconstructed via infocmp from file: 
/usr/share/terminfo/s/screen-256color
    screen-256color|GNU Screen with 256 colors,
    ...

Thank you!

-- 

  Sergei