Correctly align page's images in generic wal API

Page image should be MAXALIGN'ed because existing code could directly align
pointers in page instead of align offset from beginning of page.

Found during play with indexes as extenstion, Alexander Korotkov and me

src/backend/access/transam/generic_xlog.c

@@ -52,7 +52,9 @@ typedef struct
 	Buffer		buffer;			/* registered buffer */
 	int			flags;			/* flags for this buffer */
 	int			deltaLen;		/* space consumed in delta field */
-	char		image[BLCKSZ];	/* copy of page image for modification */
+	char		*image;			/* copy of page image for modification,
+								 * do not do it in-place to have aligned
+								 * memory chunk */
 	char		delta[MAX_DELTA_SIZE];	/* delta between page images */
 } PageData;
 
@@ -268,7 +270,15 @@ GenericXLogStart(Relation relation)
 
 	state->isLogged = RelationNeedsWAL(relation);
 	for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
+	{
+		/*
+		 * pre-alloc page's images to prevent allocation in
+		 * GenericXLogRegisterBuffer() which could be called in different
+		 * memory context(s)
+		 */
+		state->pages[i].image = palloc(BLCKSZ);
 		state->pages[i].buffer = InvalidBuffer;
+	}
 
 	return state;
 }
@@ -422,6 +432,8 @@ GenericXLogFinish(GenericXLogState *state)
 		lsn = InvalidXLogRecPtr;
 	}
 
+	for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
+		pfree(state->pages[i].image);
 	pfree(state);
 
 	return lsn;