extract_first_word: Refactor EXTRACT_DONT_COALESCE_SEPARATORS handling

extract_first_word: Refactor EXTRACT_DONT_COALESCE_SEPARATORS handling

Refactor allocation of the result string to the top, since it is
currently done in both branches of the condition.

Remove unreachable code checking for EXTRACT_DONT_COALESCE_SEPARATORS
when state == SEPARATOR (the only place where SEPARATOR is assigned to
state follows a check for EXTRACT_DONT_COALESCE_SEPARATORS that jumps to
the end of the function.)

Tested by running test-util successfully.

Follow up to: 206644aedeb8859801051ac170ec562c6a113a79