Multiline C comments in the middle of a line are not handled

Try commenting out this line of C code (assuming `set commentstring=/*\ %s\ */`):
```c
dot_filter_set(curr_view, /*visible=*/0);
```
The expected result is:
```c
/* dot_filter_set(curr_view, /1*visible=*1/0); */
```
But 64a654ef4a20db1727938338310209b6a63f60c9 produces:
```c
/* dot_filter_set(curr_view, /*visible=*/0); */
```
