Fix startLineNumber from potentially breaking.

[whitneyit]

When trying to clean my code I run into the following error:

TypeError: Cannot read property 'start' of undefined
at Object.convertDefinesAndRequires ($PWD/node_modules/amdclean/src/amdclean.js:981:64)
at Controller.estraverse.replace.enter ($PWD/node_modules/amdclean/src/amdclean.js:1218:40)
at Controller.__execute ($PWD/node_modules/estraverse/estraverse.js:399:31)
at Controller.replace ($PWD/node_modules/estraverse/estraverse.js:633:27)
at Object.replace ($PWD/node_modules/estraverse/estraverse.js:714:27)
at Object.traverseAndUpdateAst ($PWD/node_modules/amdclean/src/amdclean.js:1197:14)
at Object.clean ($PWD/node_modules/amdclean/src/amdclean.js:1271:30)
at publicAPI.clean ($PWD/node_modules/amdclean/src/amdclean.js:1667:91)
at DestroyableTransform._transform ($PWD/index.js:39:26)
at DestroyableTransform.Transform._read ($PWD/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:159:10)
at DestroyableTransform.Transform._write ($PWD/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:147:83)
at doWrite ($PWD/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:313:64)
at writeOrBuffer ($PWD/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:302:5)
at DestroyableTransform.Writable.write ($PWD/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:241:11)
at CompileStream.ondata (_stream_readable.js:528:20)
at emitOne (events.js:77:13)

I found by hardening the check of convertDefinesAndRequires with the
code in this PR prevents this error from being thrown.

[danielbodart]

I had the same error when using AMDClean on a Typescript bundle. Here is my single line version of the above:

startLineNumber = (isDefine || isRequire) && node && node.expression && node.expression.loc && node.expression.loc.start ? node.expression.loc.start.line : node && node.loc && node.loc.start ? node.loc.start.line : null;