Ensure Soundex difference() function handles empty input sanely.

 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