Silence compiler warnings for unsupported compression methods.

wrasse, at least, moans about the lack of any "return" statement
in these functions.  You'd think pretty much everything would
know that exit(1) doesn't return, but evidently not.

src/bin/pg_basebackup/bbstreamer_gzip.c

@@ -116,6 +116,7 @@ bbstreamer_gzip_writer_new(char *pathname, FILE *file,
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support gzip compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }
 
@@ -248,6 +249,7 @@ bbstreamer_gzip_decompressor_new(bbstreamer *next)
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support gzip compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }
 

src/bin/pg_basebackup/bbstreamer_lz4.c

@@ -99,6 +99,7 @@ bbstreamer_lz4_compressor_new(bbstreamer *next, bc_specification *compress)
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support lz4 compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }
 
@@ -296,6 +297,7 @@ bbstreamer_lz4_decompressor_new(bbstreamer *next)
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support lz4 compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }
 

src/bin/pg_basebackup/bbstreamer_zstd.c

@@ -117,6 +117,7 @@ bbstreamer_zstd_compressor_new(bbstreamer *next, bc_specification *compress)
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support zstd compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }
 
@@ -271,6 +272,7 @@ bbstreamer_zstd_decompressor_new(bbstreamer *next)
 	return &streamer->base;
 #else
 	pg_fatal("this build does not support zstd compression");
+	return NULL;				/* keep compiler quiet */
 #endif
 }