proc: fix stacktrace frame after runtime.sigpanic (#3638)

The first frame after sigpanic didn't execute a call so we shouldn't
decrement the PC address to look up its location.

Fixes #3634

_fixtures/panicline.go

@@ -0,0 +1,8 @@
+package main
+
+import "os"
+
+func main() {
+	fi, _ := os.Lstat("/this/path/does/not/exist")
+	fi.Size()
+}

pkg/proc/proc_test.go

@@ -6121,3 +6121,28 @@ func TestIssue3545(t *testing.T) {
 		}
 	})
 }
+
+func TestPanicLine(t *testing.T) {
+	withTestProcess("panicline", t, func(p *proc.Target, grp *proc.TargetGroup, fixture protest.Fixture) {
+		err := grp.Continue()
+		if runtime.GOOS == "darwin" && err != nil && err.Error() == "bad access" {
+			// not supported
+			return
+		}
+		assertNoError(err, t, "Continue()")
+		frames, err := proc.ThreadStacktrace(p, p.CurrentThread(), 20)
+		assertNoError(err, t, "ThreadStacktrace")
+		logStacktrace(t, p, frames)
+
+		found := false
+		for _, frame := range frames {
+			if strings.HasSuffix(frame.Call.File, "panicline.go") && frame.Call.Line == 7 {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Fatalf("could not find panicline.go:6")
+		}
+	})
+}

pkg/proc/stack.go

@@ -180,6 +180,7 @@ type stackIterator struct {
 	pc     uint64
 	top    bool
 	atend  bool
+	sigret bool
 	frame  Stackframe
 	target *Target
 	bi     *BinaryInfo
@@ -270,6 +271,7 @@ func (it *stackIterator) Next() bool {
 		return true
 	}
 
+	it.sigret = it.frame.Current.Fn != nil && it.frame.Current.Fn.Name == "runtime.sigpanic"
 	it.top = false
 	it.pc = it.frame.Ret
 	it.regs = callFrameRegs
@@ -329,7 +331,7 @@ func (it *stackIterator) newStackframe(ret, retaddr uint64) Stackframe {
 		r.Regs.AddReg(it.regs.PCRegNum, op.DwarfRegisterFromUint64(it.pc))
 	}
 	r.Call = r.Current
-	if !it.top && r.Current.Fn != nil && it.pc != r.Current.Fn.Entry {
+	if !it.top && r.Current.Fn != nil && it.pc != r.Current.Fn.Entry && !it.sigret {
 		// if the return address is the entry point of the function that
 		// contains it then this is some kind of fake return frame (for example
 		// runtime.sigreturn) that didn't actually call the current frame,