Fix support for libstemmer_c

It comes without a version.
Occurs in Wendelin.

nxdbom/__init__.py

@@ -53,7 +53,12 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
         if version is not None:
             assert ver is None
             ver = version
-        assert ver is not None, urlpath
+        if ver is None:
+            # some packages come without version in the url
+            if name in ('libstemmer_c',):
+                ver = ''
+            else:
+                assert ver is not None, urlpath
         ver = removeprefix(ver, name+'-')   # wendelin.core-2.0.alpha3-0-g6315384  -> 2.0.alpha3-0-g6315384
         if '//' in urlpath:
             url = urlpath

nxdbom/nxdbom_test.py

@@ -44,6 +44,7 @@ from os.path import dirname, exists
     ('http://www.cups.org/software/1.7.4/cups-1.7.4-source.tar.bz2',  'cups', '1.7.4'),
     ('https://github.com/unicode-org/icu/releases/download/release-58-2/icu4c-58_2-src.tgz',  'icu4c', '58_2'),
     ('https://lab.nexedi.com/bk/onlyoffice_core/repository/archive.tar.bz2?ref=8a40eb47bd80a40ecde14c223525b21852d2fc9f',  'onlyoffice_core', '8a40eb47bd80a40ecde14c223525b21852d2fc9f'),
+    ('http://snowball.tartarus.org/dist/libstemmer_c.tgz',  'libstemmer_c', None),
 ])
 def test_namever(url, nameok, verok):
     assert nxdbom.namever(url) == (nameok, verok)
@@ -394,6 +395,15 @@ url = http://downloadarchive.documentfoundation.org/libreoffice/old/5.2.4.2/rpm/
 LibreOffice                  5.2.4.2    http://downloadarchive.documentfoundation.org/libreoffice/old/5.2.4.2/rpm/x86_64/LibreOffice_5.2.4.2_Linux_x86-64_rpm.tar.gz
 """)
 
+# libstemmer_c comes without version
+case1("""\
+[libstemmer]
+recipe = slapos.recipe.cmmi
+url = http://snowball.tartarus.org/dist/libstemmer_c.tgz
+""", """\
+libstemmer_c                            http://snowball.tartarus.org/dist/libstemmer_c.tgz
+""")
+
 
 @pytest.mark.parametrize('build,bomok', testv)
 def test_bom_software(tmpdir, build, bomok):