Fix crash on missing linker flags (#23461)

Without this change running `emcc test/hello_world.c -Xlinker` will
crash with:

```
...
  File "emcc.py", line 813, in phase_setup
    state.add_link_flag(i + 1, newargs[i + 1])
                               ~~~~~~~^^^^^^^
IndexError: list index out of range
```

emcc.py

@@ -779,6 +779,11 @@ def phase_setup(options, state, newargs):
     if arg in CLANG_FLAGS_WITH_ARGS:
       skip = True
 
+    def get_next_arg():
+      if len(newargs) <= i + 1:
+        exit_with_error(f"option '{arg}' requires an argument")
+      return newargs[i + 1]
+
     if not arg.startswith('-'):
       # we already removed -o <target>, so all these should be inputs
       newargs[i] = ''
@@ -797,7 +802,7 @@ def phase_setup(options, state, newargs):
       state.add_link_flag(i, arg)
     elif arg == '-z':
       state.add_link_flag(i, newargs[i])
-      state.add_link_flag(i + 1, newargs[i + 1])
+      state.add_link_flag(i + 1, get_next_arg())
     elif arg.startswith('-z'):
       state.add_link_flag(i, newargs[i])
     elif arg.startswith('--js-library='):
@@ -810,7 +815,7 @@ def phase_setup(options, state, newargs):
       for flag_index, flag in enumerate(link_flags_to_add):
         state.add_link_flag(i + float(flag_index) / len(link_flags_to_add), flag)
     elif arg == '-Xlinker':
-      state.add_link_flag(i + 1, newargs[i + 1])
+      state.add_link_flag(i + 1, get_next_arg())
     elif arg == '-s':
       state.add_link_flag(i, newargs[i])
     elif arg == '-':
@@ -1154,7 +1159,7 @@ def check_arg(name):
         return True
       if arg == name:
         if len(newargs) <= i + 1:
-          exit_with_error("option '%s' requires an argument" % arg)
+          exit_with_error(f"option '{arg}' requires an argument")
         arg_value = newargs[i + 1]
         newargs[i] = ''
         newargs[i + 1] = ''

pyproject.toml

@@ -50,7 +50,7 @@ lint.ignore = [
   "PLW2901",
 ]
 lint.per-file-ignores."emrun.py" = [ "PLE0704" ]
-lint.mccabe.max-complexity = 49 # Recommended: 10
+lint.mccabe.max-complexity = 51 # Recommended: 10
 lint.pylint.allow-magic-value-types = [
   "bytes",
   "float",

test/test_other.py

@@ -12300,6 +12300,10 @@ def test_linker_flags_unused(self):
     err = self.run_process([EMCC, test_file('hello_world.c'), '-Wl,-static', '-Xlinker', '-static'], stderr=PIPE).stderr
     self.assertNotContained("input unused", err)
 
+  def test_linker_flags_missing(self):
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '-Xlinker'])
+    self.assertContained("emcc: error: option '-Xlinker' requires an argument", err)
+
   def test_linker_input_unused(self):
     self.run_process([EMCC, '-c', test_file('hello_world.c')])
     err = self.run_process([EMCC, 'hello_world.o', '-c', '-o', 'out.o'], stderr=PIPE).stderr