gh-137571: Protect against possible UnboundLocalError in gzip._GzipReader.read() (GH-150222)

This has not been observed in practice, but we cannot be 100% sure that
it will not happen with some weird gzip data.
(cherry picked from commit 28eac9a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Lib/gzip.py

@@ -551,10 +551,10 @@ def read(self, size=-1):
             # Read a chunk of data from the file
             if self._decompressor.needs_input:
                 buf = self._fp.read(READ_BUFFER_SIZE)
-                uncompress = self._decompressor.decompress(buf, size)
             else:
-                uncompress = self._decompressor.decompress(b"", size)
+                buf = b""
 
+            uncompress = self._decompressor.decompress(buf, size)
             if self._decompressor.unused_data != b"":
                 # Prepend the already read bytes to the fileobj so they can
                 # be seen by _read_eof() and _read_gzip_header()