Fix exception instead of failure when parsing CallExpressions

[deathcap]

Transforming parsers generated with jison with brfs would crash when static-eval attempted to parse this code:

var source = require('fs').readFileSync(require('path').normalize(x), 'utf8');

TypeError: callee.apply is not a function while parsing file: /tmp/b
    at walk (/Users/admin/games/voxeljs/static-eval/index.js:89:27)
    at walk (/Users/admin/games/voxeljs/static-eval/index.js:92:23)
    at walk (/Users/admin/games/voxeljs/static-eval/index.js:77:26)
    at walk (/Users/admin/games/voxeljs/static-eval/index.js:85:25)
    at module.exports (/Users/admin/games/voxeljs/static-eval/index.js:114:7)
    at traverse (/Users/admin/games/voxeljs/static-module/index.js:256:23)
    at walk (/Users/admin/games/voxeljs/static-module/index.js:208:13)
    at walk (/Users/admin/games/voxeljs/static-module/node_modules/falafel/index.js:49:9)
    at /Users/admin/games/voxeljs/static-module/node_modules/falafel/index.js:46:17
    at forEach (/Users/admin/games/voxeljs/static-module/node_modules/falafel/node_modules/foreach/index.js:12:16)

This PR fixes the crash, and correctly returns FAIL so it won't be transformed, as expected (it cannot be, since it is dynamic).

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Smaller (but nonsensical) test case:

var source = require('fs').readFileSync(require('path'));

require() parses as not a function, but an object { resolve: …}, and then static-eval tries to call it, causing this exception. These other cases parse correctly (not transformed, expected):

var path = require('path').normalize(x);
var source = require('fs').readFileSync(path, 'utf8');

function f() { return '/tmp/foo'; }
var source = require('fs').readFileSync(f());

only readFileSync(require…) causes the crash, which this PR fixes (by gracefully failing instead).

[nfriedly]

This bug has been biting me all morning too. I'm trying to run some tests in karma with browserify and brfs and kept getting "TypeError: Cannot read property 'apply' of undefined while parsing file..." and eventually tracked it down to that same section of code trying to run callee.apply(ctx, args) when callee is undefined