We check if the next character after `N.` is `*` before we check its
length. Using `split` on the string is cleaner and less error prone than
using indices with `find` and `substr`.
Note: this does not make `N.` mean anything, it just prevents assertion
failures. `N.` is treated the same as an unrecognized breakpoint name:
```
(lldb) breakpoint enable 1
1 breakpoints enabled.
(lldb) breakpoint enable 1.*
1 breakpoints enabled.
(lldb) breakpoint enable 1.
0 breakpoints enabled.
(lldb) breakpoint enable xyz
0 breakpoints enabled.
```
Found via LLDB fuzzers.
a6caceed8d