[lldb] Fix crash in FormatEntity for mangled-name

Check a `Block` pointer before dereferencing.

Using `function.mangled-name` led to a crash for a frame where the symbol
context had no block info. In my case, the frame's function was a system frame.

Differential Revision: https://reviews.llvm.org/D96307

GitOrigin-RevId: 7dc324aafa2b17a4f9a992b9727a3642505053a6
1 file changed