Add bounds checking for sampleTypeIndex

[jlfwong]

Wow this was surprising. As reported in #448, the simple.speedscope.json file failed import. This was surprising to me because there's a test that covers that file to ensure it imports correctly.

The file provided in #448, however, is from a version of speedscope from 5 years ago before the file had been pretty printed. It turns out that when this particular file is not pretty-printed, it's a schematically valid pprof profile.

The fix is to do some bounds checks and return null. After the change, the file imports as you'd expect after realizing its not actually a valid pprof profile.

Fixes #448

[coveralls]

coverage: 43.041% (+0.09%) from 42.952%
when pulling 87391ee on jlfwong/add-more-error-checking-for-pprof-import
into de17f12 on main.