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 
 -------------

contrib/fuzzystrmatch/fuzzystrmatch.c

@@ -728,16 +728,14 @@ _soundex(const char *instr, char *outstr)
 	Assert(instr);
 	Assert(outstr);
 
-	outstr[SOUNDEX_LEN] = '\0';
-
 	/* Skip leading non-alphabetic characters */
-	while (!isalpha((unsigned char) instr[0]) && instr[0])
+	while (*instr && !isalpha((unsigned char) *instr))
 		++instr;
 
-	/* No string left */
-	if (!instr[0])
+	/* If no string left, return all-zeroes buffer */
+	if (!*instr)
 	{
-		outstr[0] = (char) 0;
+		memset(outstr, '\0', SOUNDEX_LEN + 1);
 		return;
 	}
 
@@ -750,7 +748,7 @@ _soundex(const char *instr, char *outstr)
 		if (isalpha((unsigned char) *instr) &&
 			soundex_code(*instr) != soundex_code(*(instr - 1)))
 		{
-			*outstr = soundex_code(instr[0]);
+			*outstr = soundex_code(*instr);
 			if (*outstr != '0')
 			{
 				++outstr;
@@ -767,6 +765,9 @@ _soundex(const char *instr, char *outstr)
 		++outstr;
 		++count;
 	}
+
+	/* And null-terminate */
+	*outstr = '\0';
 }
 
 PG_FUNCTION_INFO_V1(difference);

contrib/fuzzystrmatch/sql/fuzzystrmatch.sql

@@ -6,6 +6,7 @@ SELECT soundex('hello world!');
 SELECT soundex('Anne'), soundex('Ann'), difference('Anne', 'Ann');
 SELECT soundex('Anne'), soundex('Andrew'), difference('Anne', 'Andrew');
 SELECT soundex('Anne'), soundex('Margaret'), difference('Anne', 'Margaret');
+SELECT soundex(''), difference('', '');
 
 
 SELECT levenshtein('GUMBO', 'GAMBOL');