gracefully handle elided frames (#120)

When parsing stacks we frequently encounter elided frames. The actual
stack line looks something like `...23 frames elided...`, which is
passed to `parseFuncName`, which of course is not parsable as a function
name. As a result, whenever this happens we get a useless panic like the
following:

```
panic: Failed to parse stack trace: parse function: no function found: "...23 frames elided..."
goroutine 1 [running]:
go.uber.org/goleak/internal/stack.getStackBuffer(0x1)
	/home/runner/go/pkg/mod/go.uber.org/goleak@v1.3.0/internal/stack/stacks.go:240 +0x65
go.uber.org/goleak/internal/stack.getStacks(0x1)
	/home/runner/go/pkg/mod/go.uber.org/goleak@v1.3.0/internal/stack/stacks.go:84 +0x3b
go.uber.org/goleak/internal/stack.All(...)
	/home/runner/go/pkg/mod/go.uber.org/goleak@v1.3.0/internal/stack/stacks.go:229
go.uber.org/goleak.Find({0xc0003ae880, 0x4, 0x4})
	/home/runner/go/pkg/mod/go.uber.org/goleak@v1.3.0/leaks.go:65 +0x1ea
```

This PR gracefully handles elided frames to prevent the panic.

internal/stack/stacks.go

@@ -158,6 +158,13 @@ func (p *stackParser) parseStack(line string) (Stack, error) {
 			// Just skip it.
 			continue
 		}
+		if strings.HasPrefix(line, "...") && strings.HasSuffix(line, " frames elided...") {
+			// e.g. ...23 frames elided...
+			// This indicates frames were elided from the stack trace,
+			// attempting to parse them via parseFuncName will fail resulting in a panic
+			// and a relatively useless output. Gracefully handle this.
+			continue
+		}
 
 		funcName, creator, err := parseFuncName(line)
 		if err != nil {

internal/stack/stacks_test.go

@@ -21,6 +21,7 @@
 package stack
 
 import (
+	"bytes"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -136,8 +137,8 @@ func TestCurrentCreatedBy(t *testing.T) {
 
 func TestAllLargeStack(t *testing.T) {
 	const (
-		stackDepth    = 100
-		numGoroutines = 100
+		stackDepth    = 101
+		numGoroutines = 101
 	)
 
 	var started sync.WaitGroup
@@ -163,6 +164,15 @@ func TestAllLargeStack(t *testing.T) {
 		t.Fatalf("Expected larger stack buffer")
 	}
 
+	// Also test the stack parser here to ensure it handles elided frames gracefully.
+	// We want to check this here, so that if the format of the "elided frames" message changes, we catch it.
+	// At the time of writing this test, with a stack depth of 101, we get 2 elided frames:
+	// "...2 frames elided...".
+	assert.Contains(t, string(buf), "frames elided...")
+	stacks, err := newStackParser(bytes.NewReader(buf)).Parse()
+	require.NoError(t, err)
+	assert.Greater(t, len(stacks), numGoroutines, "expect more parsed stacks than goroutines")
+
 	// Start enough goroutines so we exceed the default buffer size.
 	close(done)
 }
@@ -263,6 +273,23 @@ func TestParseStack(t *testing.T) {
 				"example.com/foo/bar.baz",
 			},
 		},
+		{
+			name: "elided frames",
+			give: joinLines(
+				"goroutine 1 [running]:",
+				"example.com/foo/bar.baz()",
+				"	example.com/foo/bar.go:123",
+				"...3 frames elided...",
+				"created by example.com/foo/bar.qux",
+				"	example.com/foo/bar.go:456",
+			),
+			id:        1,
+			state:     "running",
+			firstFunc: "example.com/foo/bar.baz",
+			funcs: []string{
+				"example.com/foo/bar.baz",
+			},
+		},
 	}
 
 	for _, tt := range tests {