Replace pointer comparisons and assignments to literal zero with NULL

While 0 is technically correct, NULL is the semantically appropriate
choice for pointers.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/aS1AYnZmuRZ8g%2B5G%40ip-10-97-1-34.eu-west-3.compute.internal

src/backend/nodes/copyfuncs.c

@@ -204,7 +204,7 @@ copyObjectImpl(const void *from)
 
 		default:
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(from));
-			retval = 0;			/* keep compiler quiet */
+			retval = NULL;		/* keep compiler quiet */
 			break;
 	}
 

src/backend/postmaster/postmaster.c

@@ -3391,7 +3391,7 @@ LaunchMissingBackgroundProcesses(void)
 			Shutdown <= SmartShutdown)
 		{
 			WalReceiverPMChild = StartChildProcess(B_WAL_RECEIVER);
-			if (WalReceiverPMChild != 0)
+			if (WalReceiverPMChild != NULL)
 				WalReceiverRequested = false;
 			/* else leave the flag set, so we'll try again later */
 		}

src/backend/utils/adt/pg_locale.c

@@ -1222,10 +1222,10 @@ pg_newlocale_from_collation(Oid collid)
 		 * Make sure cache entry is marked invalid, in case we fail before
 		 * setting things.
 		 */
-		cache_entry->locale = 0;
+		cache_entry->locale = NULL;
 	}
 
-	if (cache_entry->locale == 0)
+	if (cache_entry->locale == NULL)
 	{
 		cache_entry->locale = create_pg_locale(collid, CollationCacheContext);
 	}

src/backend/utils/adt/timestamp.c

@@ -5161,7 +5161,7 @@ interval_trunc(PG_FUNCTION_ARGS)
 							 errmsg("unit \"%s\" not supported for type %s",
 									lowunits, format_type_be(INTERVALOID)),
 							 (val == DTK_WEEK) ? errdetail("Months usually have fractional weeks.") : 0));
-					result = 0;
+					result = NULL;
 			}
 		}
 

src/interfaces/ecpg/ecpglib/prepare.c

@@ -509,7 +509,7 @@ ecpg_freeStmtCacheEntry(int lineno, int compat,
 	if (entry->ecpgQuery)
 	{
 		ecpg_free(entry->ecpgQuery);
-		entry->ecpgQuery = 0;
+		entry->ecpgQuery = NULL;
 	}
 
 	return entNo;