profile.CheckValid should return an error if location has a line with empty function (#517)

* Check if location has a line with empty function

* Include context in the error message that will help triage the error

profile/profile.go

 			}
 		}
 		for _, ln := range l.Line {
-			if f := ln.Function; f != nil {
-				if f.ID == 0 || functions[f.ID] != f {
-					return fmt.Errorf("inconsistent function %p: %d", f, f.ID)
-				}
+			f := ln.Function
+			if f == nil {
+				return fmt.Errorf("location id: %d has a line with nil function", l.ID)
+			}
+			if f.ID == 0 || functions[f.ID] != f {
+				return fmt.Errorf("inconsistent function %p: %d", f, f.ID)
 			}
 		}
 	}

profile/profile_test.go

 			mutateFn: func(p *Profile) { p.Function[0] = nil },
 			wantErr:  "profile has nil function",
 		},
+		{
+			mutateFn: func(p *Profile) { p.Location[0].Line = append(p.Location[0].Line, Line{}) },
+			wantErr:  "has a line with nil function",
+		},
 	} {
 		t.Run(tc.wantErr, func(t *testing.T) {
 			p := p.Copy()