Ensure Soundex difference() function handles empty input sanely.

fuzzystrmatch's difference() function assumes that _soundex()
always initializes its output buffer fully.  This was not so for
the case of a string containing no alphabetic characters, resulting
in unstable output and Valgrind complaints.

Fix by using memset() to fill the whole buffer in the early-exit
case.  Also make some cosmetic improvements (I didn't care for the
random switches between "instr[0]" and "*instr" notation).

Report and diagnosis by Alexander Lakhin (bug #17935).
Back-patch to all supported branches.

Discussion: https://postgr.es/m/17935-b99316aa79c18513@postgresql.org

contrib/fuzzystrmatch/expected/fuzzystrmatch.out

@@ -23,6 +23,12 @@ SELECT soundex('Anne'), soundex('Margaret'), difference('Anne', 'Margaret');
  A500    | M626    |          0
 (1 row)
 
+SELECT soundex(''), difference('', '');
+ soundex | difference 
+---------+------------
+         |          4
+(1 row)
+
 SELECT levenshtein('GUMBO', 'GAMBOL');
  levenshtein 
 -------------