transfer: fix returning init failures from `xfer_recv_shutdown_started()`

transfer: fix returning init failures from `xfer_recv_shutdown_started()`

Before this patch it returned `CURLE_FAILED_INIT` on init failures, with
the value of 2. Fix it to return `false`.

Seen with clang 18.1.8:
```
../lib/transfer.c(181,12): warning: integer constant not in range of enumerated type 'bool' [-Wassign-enum]
  181 |     return CURLE_FAILED_INIT;
      |            ^
../lib/transfer.c(181,12): warning: implicit conversion from enumeration type 'CURLcode' to different enumeration type 'bool' [-Wenum-conversion]
  181 |     return CURLE_FAILED_INIT;
      |     ~~~~~~ ^~~~~~~~~~~~~~~~~
../lib/transfer.c(183,12): warning: integer constant not in range of enumerated type 'bool' [-Wassign-enum]
  183 |     return CURLE_FAILED_INIT;
      |            ^
../lib/transfer.c(183,12): warning: implicit conversion from enumeration type 'CURLcode' to different enumeration type 'bool' [-Wenum-conversion]
  183 |     return CURLE_FAILED_INIT;
      |     ~~~~~~ ^~~~~~~~~~~~~~~~~
```

Follow-up to 35bf766280a77cde3055e0f4e249ab02a0dcd275 #14253

Closes #16170