Initialize variables to placate compiler.

Since commit 012460e, some compilers have been warning that a
couple of variables may be used uninitialized.  There doesn't
appear to be any actual risk, so let's just initialize these
variables to 0 to silence the compiler warnings.

Discussion: https://postgr.es/m/20240317192927.GA3978212%40nathanxps13

Author: nathan-bossart

src/backend/commands/statscmds.c

@@ -606,7 +606,7 @@ AlterStatistics(AlterStatsStmt *stmt)
 	bool		repl_null[Natts_pg_statistic_ext];
 	bool		repl_repl[Natts_pg_statistic_ext];
 	ObjectAddress address;
-	int			newtarget;
+	int			newtarget = 0;
 	bool		newtarget_default;
 
 	/* -1 was used in previous versions for the default setting */

src/backend/commands/tablecmds.c

@@ -8711,7 +8711,7 @@ ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMOD
 static ObjectAddress
 ATExecSetStatistics(Relation rel, const char *colName, int16 colNum, Node *newValue, LOCKMODE lockmode)
 {
-	int			newtarget;
+	int			newtarget = 0;
 	bool		newtarget_default;
 	Relation	attrelation;
 	HeapTuple	tuple,