Fix infinite loop in split_by_comma_or_colon(). (#1332)

* Fix infinite loop in `split_by_comma_or_colon()`.

The mere presence of a `,` or a `:` in the input string would make the
function enter an infinite loop, effectively freezing the program until
all available memory was exhausted. The string position in the loop was
not incremented past the `,` or the `;` character that had just been
read.

The function is currently used only in the `--exclude-libs` option. The
freeze is reproducible with:

```bash
$ mold --exclude-libs foo,bar
```

The longer the string after the first `,` or `:` is, the faster memory
exhaustion is reached.

* Skip empty parts in `split_by_comma_or_colon()`.

src/cmdline.cc

@@ -422,14 +422,16 @@ static std::vector<std::string_view>
 split_by_comma_or_colon(std::string_view str) {
   std::vector<std::string_view> vec;
 
-  for (;;) {
+  while (!str.empty()) {
     i64 pos = str.find_first_of(",:");
     if (pos == str.npos) {
       vec.push_back(str);
       break;
     }
-    vec.push_back(str.substr(0, pos));
-    str = str.substr(pos);
+    if (pos > 0) {
+      vec.push_back(str.substr(0, pos));
+    }
+    str = str.substr(pos + 1);
   }
   return vec;
 }