Add new function dsa_allocate0.

This does the same thing as dsa_allocate, except that the memory
is guaranteed to be zero-filled on return.

Dilip Kumar, adjusted by me.

src/backend/utils/mmgr/dsa.c

@@ -755,6 +755,22 @@ dsa_allocate(dsa_area *area, Size size)
 	return alloc_object(area, size_class);
 }
 
+/*
+ * As dsa_allocate, but zeroes the allocated memory.
+ */
+dsa_pointer
+dsa_allocate0(dsa_area *area, Size size)
+{
+	dsa_pointer dp;
+	char	   *object;
+
+	dp = dsa_allocate(area, size);
+	object = dsa_get_address(area, dp);
+	memset(object, 0, size);
+
+	return dp;
+}
+
 /*
  * Free memory obtained with dsa_allocate.
  */