Suppress various new compiler warnings.

Compilers that don't understand that elog(ERROR) doesn't return
issued warnings here.  In the cases in libpq_pipeline.c, we were
not exactly helping things by failing to mark pg_fatal() as noreturn.

Per buildfarm.

src/backend/access/common/detoast.c

@@ -487,6 +487,7 @@ toast_decompress_datum(struct varlena *attr)
 			return lz4_decompress_datum(attr);
 		default:
 			elog(ERROR, "invalid compression method id %d", cmid);
+			return NULL;		/* keep compiler quiet */
 	}
 }
 
@@ -518,6 +519,7 @@ toast_decompress_datum_slice(struct varlena *attr, int32 slicelength)
 			return lz4_decompress_datum_slice(attr, slicelength);
 		default:
 			elog(ERROR, "invalid compression method id %d", cmid);
+			return NULL;		/* keep compiler quiet */
 	}
 }
 

src/backend/access/common/toast_compression.c

@@ -135,6 +135,7 @@ lz4_compress_datum(const struct varlena *value)
 {
 #ifndef USE_LZ4
 	NO_LZ4_SUPPORT();
+	return NULL;				/* keep compiler quiet */
 #else
 	int32		valsize;
 	int32		len;
@@ -177,6 +178,7 @@ lz4_decompress_datum(const struct varlena *value)
 {
 #ifndef USE_LZ4
 	NO_LZ4_SUPPORT();
+	return NULL;				/* keep compiler quiet */
 #else
 	int32		rawsize;
 	struct varlena *result;
@@ -209,6 +211,7 @@ lz4_decompress_datum_slice(const struct varlena *value, int32 slicelength)
 {
 #ifndef USE_LZ4
 	NO_LZ4_SUPPORT();
+	return NULL;				/* keep compiler quiet */
 #else
 	int32		rawsize;
 	struct varlena *result;

src/include/access/toast_compression.h

@@ -69,6 +69,7 @@ GetCompressionMethodName(char method)
 			return "lz4";
 		default:
 			elog(ERROR, "invalid compression method %c", method);
+			return NULL;		/* keep compiler quiet */
 	}
 }
 

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -60,6 +60,7 @@ exit_nicely(PGconn *conn)
  */
 #define pg_fatal(...) pg_fatal_impl(__LINE__, __VA_ARGS__)
 static void
+pg_attribute_noreturn()
 pg_fatal_impl(int line, const char *fmt,...)
 {
 	va_list		args;
@@ -570,6 +571,7 @@ test_pipelined_insert(PGconn *conn, int n_rows)
 
 			default:
 				pg_fatal("invalid state");
+				sql = NULL;		/* keep compiler quiet */
 		}
 
 		pg_debug("sending: %s\n", sql);
@@ -679,8 +681,8 @@ test_pipelined_insert(PGconn *conn, int n_rows)
 						break;
 					case BI_DONE:
 						/* unreachable */
-						description = "";
-						abort();
+						pg_fatal("unreachable state");
+						cmdtag = NULL;	/* keep compiler quiet */
 				}
 
 				if (PQresultStatus(res) != status)