Issue #28774: Fix start/end pos in unicode_encode_ucs1().

Fix error position of the unicode error in ASCII and Latin1
encoders when a string returned by the error handler contains multiple
non-encodable characters (non-ASCII for the ASCII codec, characters out
of the U+0000-U+00FF range for Latin1).
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e88a126..2bf48b7 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -6798,7 +6798,7 @@
                     goto onError;
 
                 /* subtract preallocated bytes */
-                writer.min_size -= 1;
+                writer.min_size -= newpos - collstart;
 
                 if (PyBytes_Check(rep)) {
                     /* Directly copy bytes result to output. */
@@ -6835,7 +6835,7 @@
                             ch = PyUnicode_READ_CHAR(rep, i);
                             if (ch >= limit) {
                                 raise_encode_exception(&exc, encoding, unicode,
-                                                       pos, pos+1, reason);
+                                                       collstart, collend, reason);
                                 goto onError;
                             }
                             *str = (char)ch;