Save segfault when creating new folder on Mac witj Julia 0.6

When opening just a single window, then trying to save the plot, if I create a new folder (using the 'New folder' button), Julia segfaults

**Repro**

```
using PyPlot
PyPlot.figure()
```

then 'save', 'new folder'

**Result**

```
signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
strlen at /usr/lib/system/libsystem_c.dylib (unknown line)
Allocations: 3764370 (Pool: 3763011; Big: 1359); GC: 6
Segmentation fault: 11
```

**Extra info**

```
Process: julia [57129]
Path: /usr/local/bin/julia
Identifier: julia
Version: ???
Code Type: X86-64 (Native)
Parent Process: bash [57017]
Responsible: julia [57129]
User ID: 501
Date/Time: 2017-10-05 10:00:07.999 -0700
OS Version: Mac OS X 10.12.6 (16G29)

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY

Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: exc handler [0]
```

Reproduce the issue on macOS 10.12.6 with Julia 0.6 by running the provided PyPlot.figure() example, then using Save and New folder. Trace the save-dialog path that triggers the segmentation fault; done means creating a folder from the plot-save flow no longer crashes Julia.

Scritto dal modello di indicizzazione a partire dal testo della issue.