build: fix macOS --enable-modules build

Apple's nm implementation includes empty lines in the output that are not
found in GNU binutils.  This confuses scripts/undefsym.py, though it did
not confuse the scripts/undefsym.sh script that it replaced.  To fix
this, ignore lines that do not have two fields.

Reported-by: Emmanuel Blot <eblot.ml@gmail.com>
Tested-by: Emmanuel Blot <eblot.ml@gmail.com>
Fixes: 604f3e4e90 ("meson: Convert undefsym.sh to undefsym.py", 2020-09-08)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.cirrus.yml

@@ -40,7 +40,7 @@ macos_xcode_task:
   script:
     - mkdir build
     - cd build
-    - ../configure --extra-cflags='-Wno-error=deprecated-declarations'
+    - ../configure --extra-cflags='-Wno-error=deprecated-declarations' --enable-modules
                    --enable-werror --cc=clang || { cat config.log meson-logs/meson-log.txt; exit 1; }
     - gmake -j$(sysctl -n hw.ncpu)
     - gmake check V=1

scripts/undefsym.py

@@ -15,8 +15,7 @@ def filter_lines_set(stdout, from_staticlib):
     linesSet = set()
     for line in stdout.splitlines():
         tokens = line.split(b' ')
-        if len(tokens) >= 1:
-            if len(tokens) > 1:
+        if len(tokens) >= 2:
             if from_staticlib and tokens[1] == b'U':
                 continue
             if not from_staticlib and tokens[1] != b'U':