Suppress uninitialized-variable warning in new SCRAM code.

While we generally don't sweat too much about "may be used uninitialized"
warnings from older compilers, I noticed that there's a fair number of
buildfarm animals that are producing such a warning *only* for this
variable.  So it seems worth silencing.

src/backend/libpq/auth.c

@@ -865,7 +865,7 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 	StringInfoData sasl_mechs;
 	int			mtype;
 	StringInfoData buf;
-	void	   *scram_opaq;
+	void	   *scram_opaq = NULL;
 	char	   *output = NULL;
 	int			outputlen = 0;
 	char	   *input;