extract-word: Check for early bail out before inspecting separators

It's a pretty small optimization but doesn't hurt...

Tested with test-extract-word.

src/basic/extract-word.c

@@ -39,13 +39,13 @@ int extract_first_word(const char **p, char **ret, const char *separators, Extra
         assert(p);
         assert(ret);
 
-        if (!separators)
-                separators = WHITESPACE;
-
         /* Bail early if called after last value or with no input */
         if (!*p)
                 goto finish_force_terminate;
 
+        if (!separators)
+                separators = WHITESPACE;
+
         /* Parses the first word of a string, and returns it in
          * *ret. Removes all quotes in the process. When parsing fails
          * (because of an uneven number of quotes or similar), leaves