Fix invalid pointer-to-char comparison in FBProcessFetcher.m
Fix invalid pointer-to-char comparison in FBProcessFetcher.m

Summary:
I'm working on getting RTC's iOS stack to build with Clang 10; I uncovered the
following build error:

```
fbobjc/EndToEndTests/FBSimulatorControl/src/FBControlCore/Processes/FBProcessFetcher.m:114:29: error: comparing a pointer to a null character constant; did you mean to compare to NULL? [-Werror,-Wpointer-compare]
  while (currentPosition != '\0') {
                            ^~~~
                            (void *)0
1 error generated.
```

The local context looks like string parsing to me, so I think the intention is
to compare the character at `currentPosition` to `'\0'` rather than to compare
the pointer to `NULL` so I've added the necessary dereference operator.

Reviewed By: smeenai

Differential Revision: D20124054

fbshipit-source-id: edb743325ac5b6230a1717c229da980e39dc4263

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl