Fix the refleak from test_unicode.

Fix the refleak from test_unicode.
Backport 42973 (lots of whitespace changes intermixed):

 - Reindent a confusingly indented piece of code (no intended code changes
    there)
 - Add missing DECREFs of inner-scope 'temp' variable
 - Add various missing DECREFs by changing 'return NULL' into 'goto onError'
 - Avoid double DECREF when last _PyUnicode_Resize() fails

Coverity found one of the missing DECREFs, but oddly enough not the others.