Fix staticcheck warning SA6004 that was recently added in the tool (#405)

test.sh

 # All packages.
 PKG=$(go list ./...)
 
-staticcheck $PKG
+staticcheck -ignore '
+github.com/google/pprof/internal/binutils/binutils_test.go:SA6004
+github.com/google/pprof/internal/driver/svg.go:SA6004
+github.com/google/pprof/internal/report/source_test.go:SA6004
+github.com/google/pprof/profile/filter_test.go:SA6004
+' $PKG
 unused $PKG
 
 # Packages that have any tests.