context: shada_encode_regs(): init WriteMergerState #10637

- Check shada_pack_xx() result, abort on failure.
- Use xcalloc() to zero-initialize `wms`, fixes below ASAN failure.

ASAN failure (running vim_spec.lua in a loop):

    ../src/nvim/shada.c:1773:13: runtime error: load of value 224, which is not a valid value for type 'bool'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/nvim/shada.c:1773:13

Adding an explicit check just before the ASAN failed line:

    case kSDItemRegister: {
      if (*((int *)(void *)&entry.data.reg.is_unnamed) != 1
          && *((int *)(void *)&entry.data.reg.is_unnamed) != 0) {
        abort();

    (gdb) p entry.data.reg
    +p entry.data.reg
    $5 = {name = 49 '1', type = kMTLineWise, contents = 0x60200000c250, is_unnamed = true, contents_size = 1, width = 0, additional_data = 0x0}
    (gdb) p *((uint8_t *)(void *)&entry.data.reg.is_unnamed)
    +p *((uint8_t *)(void *)&entry.data.reg.is_unnamed)
    $7 = 2049

0 files changed, 0 insertions, 0 deletions