check for NULL when parsing strings

[JerrySievert]

to reproduce the issue:

$ build/duklub repl.js

fd = uv.fs_open('./b.js', 'r', 0644);
uv.fs_read(fd, 1024, 0).toString();

before the change, an error trying to parse an empty string (NULL), after the change, expected behavior.

[JerrySievert]

i'd still recommend checking for the validity of the pointer for 0 or pointer overflows, just to be safe, then checking against *buffer or as you'd like buffer[0] (both amount to the same thing, but are coding style).

i'll happily change the second part if you'd prefer.

[creationix]

Sounds good, change the style to be more consistent with the rest of the code and I'll take it. I doubt the one extra null check will hurt any real performance.