Fix GH-9829: Bug in refactoring Windows shmat() function

`shm->addr` cannot be `NULL` here, so the whole check is completely
pointless.

Given that this is merely a clean-up and not an actual bug fix, we do
without a NEWS entry.

Closes GH-9873.

Author: cmb69

TSRM/tsrm_win32.c

@@ -691,13 +691,6 @@ TSRM_API void *shmat(int key, const void *shmaddr, int flags)
 	}
 
 	shm->addr = shm->descriptor + sizeof(shm->descriptor);
-
-	if (NULL == shm->addr) {
-		int err = GetLastError();
-		SET_ERRNO_FROM_WIN32_CODE(err);
-		return (void*)-1;
-	}
-
 	shm->descriptor->shm_atime = time(NULL);
 	shm->descriptor->shm_lpid  = getpid();
 	shm->descriptor->shm_nattch++;