Make --ftrace more crash proof with foreign code

Sometimes we need to interact with code that wasn't compiled using
`-fno-omit-frame-pointer`. For example, if a function pointer gets
passed and called by a foreign function, linked by cosmo_dlopen().
Function call tracing will now detect backtrace pointer corruption
and simply reduce the indentation level back to zero, as a result.

libc/runtime/ftracer.c

@@ -51,7 +51,7 @@ static struct CosmoFtrace g_ftrace;
 
 __funline int GetNestingLevelImpl(struct StackFrame *frame) {
   int nesting = -2;
-  while (frame) {
+  while (frame && !kisdangerous(frame)) {
     ++nesting;
     frame = frame->next;
   }