Try to fix spurious DSM failures on Windows.

Apparently, Windows can sometimes return an error code even when the
operation actually worked just fine.  Rearrange the order of checks
according to what appear to be the best practices in this area.

Amit Kapila

src/backend/storage/ipc/dsm_impl.c

@@ -693,6 +693,15 @@ dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size,
 								 size_high, 	/* Upper 32 bits of size */
 								 size_low,		/* Lower 32 bits of size */
 								 name);
+		if (!hmap)
+		{
+			_dosmaperr(GetLastError());
+			ereport(elevel,
+					(errcode_for_dynamic_shared_memory(),
+					 errmsg("could not create shared memory segment \"%s\": %m",
+							name)));
+			return false;
+		}
 		_dosmaperr(GetLastError());
 		if (errno == EEXIST)
 		{
@@ -711,16 +720,15 @@ dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size,
 		hmap = OpenFileMapping(FILE_MAP_WRITE | FILE_MAP_READ,
 							   FALSE,		/* do not inherit the name */
 							   name);		/* name of mapping object */
-		_dosmaperr(GetLastError());
-	}
-
-	if (!hmap)
-	{
-		ereport(elevel,
-				(errcode_for_dynamic_shared_memory(),
-				 errmsg("could not open shared memory segment \"%s\": %m",
-					name)));
-		return false;
+		if (!hmap)
+		{
+			_dosmaperr(GetLastError());
+			ereport(elevel,
+					(errcode_for_dynamic_shared_memory(),
+					 errmsg("could not open shared memory segment \"%s\": %m",
+							name)));
+			return false;
+		}
 	}
 
 	/* Map it. */