Allow profiles with no samples

A previous commit tried to eliminate a panic when reading an empty
profile by stopping support for profiles with no samples. That is
unnecessary and it can obstruct some testing.

The panic was caused by the legacy parser returning a nil profile and
no error when processing an empty profile. Detect that situation and
generate a proper profile instead.

Tested by running 'pprof /dev/null'
Add testcase for profile parsing errors

internal/driver/driver.go

 		}
 	}
 
-	if len(p.Sample) == 0 {
-		return fmt.Errorf("profile is empty")
-	}
-
 	if err := aggregate(p, vars); err != nil {
 		return err
 	}

profile/legacy_profile.go

 func parseHeap(b []byte) (p *Profile, err error) {
 	s := bufio.NewScanner(bytes.NewBuffer(b))
 	if !s.Scan() {
-		return nil, s.Err()
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+		return nil, errUnrecognized
 	}
 	p = &Profile{}
 
 func parseContention(b []byte) (p *Profile, err error) {
 	s := bufio.NewScanner(bytes.NewBuffer(b))
 	if !s.Scan() {
-		return nil, s.Err()
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+		return nil, errUnrecognized
 	}
 	line := s.Text()
 
 
 		addrs = append(addrs, parseHexAddresses(line)...)
 	}
-	if s.Err() != nil {
-		return "", nil, s.Err()
+	if err := s.Err(); err != nil {
+		return "", nil, err
 	}
 	if sameAsPrevious {
 		return line, nil, nil

profile/profile_test.go

 	"path/filepath"
 	"reflect"
 	"regexp"
+	"strings"
 	"testing"
 
 	"github.com/google/pprof/internal/proftest"
 	}
 }
 
+func TestParseError(t *testing.T) {
+
+	testcases := []string{
+		"",
+		"garbage text",
+		"\x1f\x8b", // truncated gzip header
+		"\x1f\x8b\x08\x08\xbe\xe9\x20\x58\x00\x03\x65\x6d\x70\x74\x79\x00\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00", // empty gzipped file
+	}
+
+	for i, input := range testcases {
+		_, err := Parse(strings.NewReader(input))
+		if err == nil {
+			t.Errorf("got nil, want error for input #%d", i)
+		}
+	}
+}
+
 // leaveTempfile leaves |b| in a temporary file on disk and returns the
 // temp filename. This is useful to recover a profile when the test
 // fails.