Fix unchecked return value from strdup

The pg_dump compression was using strdup() instead of pg_strdup()
and failed to check the returned pointer for out-of-memory before
dereferencing it. Fix by using pg_strdup() instead which probably
was the intention here in the original patch.

Backpatch to v16 where pg_dump compression was introduced.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CC661D60-6F4C-474D-B9CF-E789ACA3CEFC@yesql.se
Backpatch-through: 16

src/bin/pg_dump/compress_io.c

@@ -249,7 +249,7 @@ InitDiscoverCompressFileHandle(const char *path, const char *mode)
 
 	Assert(strcmp(mode, PG_BINARY_R) == 0);
 
-	fname = strdup(path);
+	fname = pg_strdup(path);
 
 	if (hasSuffix(fname, ".gz"))
 		compression_spec.algorithm = PG_COMPRESSION_GZIP;