Fix geometry decoding

It would always return the empty bytestring.

I'm guessing that the correct thing was written first, and was then
broken during refactoring.
Since the postgis tests don't run in CI yet (#558), it got missed.

gel/protocol/codecs/codecs.pyx

@@ -1164,13 +1164,17 @@ cdef geometry_encode(pgproto.CodecContext settings, WriteBuffer buf, obj):
 cdef geometry_decode(pgproto.CodecContext settings, FRBuffer *buf):
     cdef:
         object result
+        ssize_t buf_len
+
     # Just wrap the bytes into a named tuple with a single field `wkb`
     descriptor = datatypes.record_desc_new(
         ('wkb',), <object>NULL, <object>NULL)
     result = datatypes.namedtuple_new(
         datatypes.namedtuple_type_new(descriptor))
 
-    elem = PyBytes_FromStringAndSize(frb_read_all(buf), buf.len)
+    # frb_read_all adjusts buf.len, so we need to read it out first
+    buf_len = buf.len
+    elem = PyBytes_FromStringAndSize(frb_read_all(buf), buf_len)
     cpython.Py_INCREF(elem)
     cpython.PyTuple_SET_ITEM(result, 0, elem)