Fix unportable disregard of alignment requirements in RADIUS code.

The compiler is entitled to store a char[] local variable with no
particular alignment requirement.  Our RADIUS code cavalierly took such
a local variable and cast its address to a struct type that does have
alignment requirements.  On an alignment-picky machine this would lead
to bus errors.  To fix, declare the local variable honestly, and then
cast its address to char * for use in the I/O calls.

Given the lack of field complaints, there must be very few if any
people affected; but nonetheless this is a clear portability issue,
so back-patch to all supported branches.

Noted while looking at a Coverity complaint in the same code.

src/backend/libpq/auth.c

@@ -2355,14 +2355,16 @@ CheckCertAuth(Port *port)
  */
 
 /*
- * RADIUS authentication is described in RFC2865 (and several
- * others).
+ * RADIUS authentication is described in RFC2865 (and several others).
  */
 
 #define RADIUS_VECTOR_LENGTH 16
 #define RADIUS_HEADER_LENGTH 20
 #define RADIUS_MAX_PASSWORD_LENGTH 128
 
+/* Maximum size of a RADIUS packet we will create or accept */
+#define RADIUS_BUFFER_SIZE 1024
+
 typedef struct
 {
 	uint8		attribute;
@@ -2376,6 +2378,8 @@ typedef struct
 	uint8		id;
 	uint16		length;
 	uint8		vector[RADIUS_VECTOR_LENGTH];
+	/* this is a bit longer than strictly necessary: */
+	char		pad[RADIUS_BUFFER_SIZE - RADIUS_VECTOR_LENGTH];
 } radius_packet;
 
 /* RADIUS packet types */
@@ -2392,9 +2396,6 @@ typedef struct
 /* RADIUS service types */
 #define RADIUS_AUTHENTICATE_ONLY	8
 
-/* Maximum size of a RADIUS packet we will create or accept */
-#define RADIUS_BUFFER_SIZE 1024
-
 /* Seconds to wait - XXX: should be in a config variable! */
 #define RADIUS_TIMEOUT 3
 
@@ -2429,10 +2430,12 @@ CheckRADIUSAuth(Port *port)
 {
 	char	   *passwd;
 	char	   *identifier = "postgresql";
-	char		radius_buffer[RADIUS_BUFFER_SIZE];
-	char		receive_buffer[RADIUS_BUFFER_SIZE];
-	radius_packet *packet = (radius_packet *) radius_buffer;
-	radius_packet *receivepacket = (radius_packet *) receive_buffer;
+	radius_packet radius_send_pack;
+	radius_packet radius_recv_pack;
+	radius_packet *packet = &radius_send_pack;
+	radius_packet *receivepacket = &radius_recv_pack;
+	char	   *radius_buffer = (char *) &radius_send_pack;
+	char	   *receive_buffer = (char *) &radius_recv_pack;
 	int32		service = htonl(RADIUS_AUTHENTICATE_ONLY);
 	uint8	   *cryptvector;
 	int			encryptedpasswordlen;