tidy-html5:tidy_fuzzer: Heap-buffer-overflow in prvTidyEncodeCharToUTF8Bytes

[ddkilzer]

https://issues.oss-fuzz.com/issues/42498297

Found by oss-fuzz.

Fixes potential out-of-bounds write in both NormalizeSpaces() and DowngradeTypography(). Adds assert() statements to catch more bugs with fuzzing.

• src/clean.c: (NormalizeSpaces):
  (DowngradeTypography):

• Use a temporary buffer when calling PutUTF8() to avoid a heap buffer overflow write and to avoid clobbering data in-place.
• Handle all possible return values after calling PutUTF8().

• src/utf8.c: (DecodeUTF8BytesToChar):
  (GetUTF8):
  (PutUTF8):

• Add assert() statements to catch bugs during fuzzing.