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/intrin/describebacktrace.c

@@ -17,6 +17,8 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/intrin/describebacktrace.internal.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/intrin/weaken.h"
 #include "libc/log/libfatal.internal.h"
 #include "libc/nexgen32e/stackframe.h"
 
@@ -28,6 +30,9 @@ dontinstrument const char *(DescribeBacktrace)(char buf[N],
   char *pe = p + N;
   bool gotsome = false;
   while (fr) {
+    if (_weaken(kisdangerous) && _weaken(kisdangerous)(fr)) {
+      break;
+    }
     if (p + 16 + 1 + 1 <= pe) {
       if (gotsome) {
         *p++ = ' ';

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;
   }