sb-sprof: Do not record implausibly short traces

It occasionally happens that SIGPROF-HANDLER runs in a situation in
which it cannot walk the stack beyond the initial frame.

Recording such degenerate traces would disrupt certain report kinds
such as flamegraphs. In addition, this seems to happen rarely enough
that just dropping the traces shouldn't really affect statistics-based
reports.

contrib/sb-sprof/record.lisp

@@ -397,6 +397,14 @@ EXPERIMENTAL: Interface subject to change."
  (setf (values ok pc-ptr fp)
  (sb-di::x86-call-context fp))
  (unless ok
+ ;; If we fail to walk the stack beyond the
+ ;; initial frame, there is likely something
+ ;; wrong. Undo the trace start marker and the
+ ;; one sample we already recorded.
+ (when (zerop i)
+ (decf (samples-index samples)
+ (+ +elements-per-trace-start+
+ (* +elements-per-sample+ (1+ i)))))
  (return))))))
  ;; Reset thread-local allocation counter before interrupts
  ;; are enabled.