BUG: Handle message 18 missing VCP info (Fixes Unidata#3413)

At some point the fixed VCP info in message type 18 was converted to a
spare field. This still parses fine when filled with 0's, but apparently
some sites are now sending out some random bytes in this location, which
can break our parser.

src/metpy/io/nexrad.py

@@ -545,12 +545,17 @@ def _decode_msg18(self, msg_hdr):
                 attr = f'VCPAT{num}'
                 dat = self.rda[attr]
                 vcp_hdr = self.vcp_fmt.unpack_from(dat, 0)
-                off = self.vcp_fmt.size
-                els = []
-                for _ in range(vcp_hdr.num_el_cuts):
-                    els.append(self.vcp_el_fmt.unpack_from(dat, off))
-                    off += self.vcp_el_fmt.size
-                self.rda[attr] = vcp_hdr._replace(els=els)
+                # At some point these got changed to spares, so only try to parse the rest if
+                # it looks like the right data.
+                if vcp_hdr.num == num and 0 < 2 * vcp_hdr.size_hw <= len(dat):
+                    off = self.vcp_fmt.size
+                    els = []
+                    for _ in range(vcp_hdr.num_el_cuts):
+                        els.append(self.vcp_el_fmt.unpack_from(dat, off))
+                        off += self.vcp_el_fmt.size
+                    self.rda[attr] = vcp_hdr._replace(els=els)
+                else:  # Otherwise this is just spare and we should dump
+                    self.rda.pop(attr)
 
     msg31_data_hdr_fmt = NamedStruct([('stid', '4s'), ('time_ms', 'L'),
                                       ('date', 'H'), ('az_num', 'H'),

src/metpy/static-data-manifest.txt

@@ -5,6 +5,7 @@ CAM_test.nc ab62e6dca907c34eb44b842c67eb3eb817127306feb3524fa6d83670b628727c
 GFS_global.nc feae73f72340ee9e8b04fca92f182638a99316ad7262152809b7ccb9b6691e10
 GFS_test.nc b69ae13179428667f6bc14dada1d5f9af4d3737b2b76a79a6001664e1525df3c
 HI-REGIONAL_4km_3.9_20160616_1715.gini 30896dda51c9f933027d8086f0a86543efce12ea90a52981eccbce2e0ce1529e
+KJKL_20240227_102059 9344358cb53f2f9449b5a984d40a7dee8048fe6ecbb7fe219ab2420f8ee18776
 KICX_20170712_1458 94bd4f795832f056f7489a5562acf76de9a9cab1694549562cc3154abb22527c
 KLTX20050329_100015.gz cad6ad8df707ad63c9ddb7306de869186241dd821517daf21ae0a80f4ce0a58d
 KTLX19990503_235621.gz 7a097251bb7a15dbcdec75812812e41a86c5eb9850f55c3d91d120c2c61e046e

tests/io/test_nexrad.py

@@ -109,6 +109,12 @@ def test_msg15():
     assert f.clutter_filter_map['datetime'] == datetime(2013, 5, 19, 5, 15, 0, 0)
 
 
+def test_msg18_novcps():
+    """Check handling of message type 18 with VCP info now spares does not crash."""
+    f = Level2File(get_test_data('KJKL_20240227_102059', as_file_obj=False))
+    assert 'VCPAT11' not in f.rda
+
+
 def test_single_chunk(caplog):
     """Check that Level2File copes with reading a file containing a single chunk."""
     # Need to override the test level set above