Crash when emitting source maps for skipped nodes (e.g. 'catch' blocks)

[mhegazy]

Code:

//@sourceMap: true
try {
// ...
} finally {
// N.B. No 'catch' block
}

Call stack:

  TypeError: Cannot read property 'filename' of undefined

[vladima]

Cannot reproduce on the latest bits. Added test fee596b to capture the problem if it will reappear in the future