segfault if process title is not "julia"

_clean install of PyPlot v2.9.0 on Julia 1.5.3, MacOS. Python 3.8.2 was preinstalled but I never launched it before_

This runs fine:
```julia
fons@rekenmachine ~ % julia --banner=no

julia> using PyPlot

julia> plot(1:3,1:3);

julia>
```

This segfaults:
```julia
fons@rekenmachine ~ % julia --banner=no

julia> Sys.set_process_title("asdfasdf")

julia> using PyPlot

julia> plot(1:3,1:3);

signal (11): Segmentation fault: 11
in expression starting at REPL[3]:1
_ZNK7QScreen6handleEv at /Users/fons/.julia/conda/3/lib/libQt5Gui.5.9.7.dylib (unknown line)
Allocations: 12180057 (Pool: 12175774; Big: 4283); GC: 11
zsh: segmentation fault julia
```

Reproduce the issue with Julia 1.5.3, PyPlot v2.9.0, Python 3.8.2, and macOS using the two REPL sequences in the report. Compare the working and crashing cases, then investigate the reported Qt5Gui QScreen::handle stack frame. Done means plotting no longer segfaults after setting the process title.

由索引模型根据 Issue 内容生成。