zstd: prevent potential free() of uninitialized memory

This is a cherry pick of an upstream fix. The free() of uninitialed
memory could likely only occur if a malloc() inside zstd fails.

The patched functions aren't currently used by Mercurial. But I don't
like leaving footguns sitting around.

contrib/python-zstandard/c-ext/compressor.c

@@ -258,6 +258,9 b' static PyObject* ZstdCompressor_copy_str'
 		return NULL;
 	}
 
+	/* Prevent free on uninitialized memory in finally. */
+	output.dst = NULL;
+
 	cstream = CStream_from_ZstdCompressor(self, sourceSize);
 	if (!cstream) {
 		res = NULL;

contrib/python-zstandard/c-ext/decompressor.c

@@ -165,6 +165,9 b' static PyObject* Decompressor_copy_strea'
 		return NULL;
 	}
 
+	/* Prevent free on uninitialized memory in finally. */
+	output.dst = NULL;
+
 	dstream = DStream_from_ZstdDecompressor(self);
 	if (!dstream) {
 		res = NULL;