windows: fix issues detected by clang-tidy, and some more

- digest_sspi: memory leak.
- digest_sspi: free buffers on `calloc()` fail.
  (not detected by clang-tidy)
- schannel_verify: avoid a `NULL` `alt_name_info`.
- schannel: fix potential `NULL` deref for `backend->cred`.
- schannel: fix uninitialized result value.
  Follow-up to 7f4c358541 #3197
- schannel: drop unused assigment.
- tool_doswin: drop unused assigment.
- testutil: fix memory leak on error.
- testutil: fix memory leak on non-error.
  (not detected by clang-tidy)

Cherry-picked from #16764
Closes #16777

src/tool_doswin.c

@@ -177,7 +177,6 @@ SANITIZEcode sanitize_file_name(char **const sanitized, const char *file_name,
 
     if(clip) {
       *clip = '\0';
-      len = clip - target;
     }
   }