Crash when copying style with same name from storage to script and vice versa

[Paranoja]

Aegisub crashes when trying to copy style from storage to a script if the style with the same name already exists in the script or vice versa.

Steps to reproduce:

1. Open Aegisub.
2. Go to Subtitles -> Styles Manager. If no styles with the same names are in both 'Storage' and 'Current Script' lists, create two styles with the same names - one in 'Current Script' and one in 'Storage' lists.
3. Try to copy style from 'Storage' to 'Current Script' or vice versa.

Result:
Aegisub crashes with message:

Oops, Aegisub has crashed!

I have tried to emergency-save a copy of your file, and a crash log file has been generated.

You can find the emergency-saved file in:
/home/max/.aegisub-2.1/recovered/.RECOVER.ass

If you submit the crash log to the Aegisub team, we will investigate the problem and attempt to fix it. You can find the crashlog in:
/home/max/.aegisub-2.1/crashlog.txt

Aegisub will now close.

Expected result:
Aegisub informs user about already existing style and asks user whether it should overwrite it or not.

[verm]

Crashes for me on FreeBSD too.

[plorkyeran]

(In [3960]) Fix undefined behavior sometimes resulting in a crash when copying styles to or from storage when the style name already exists. Patch by Paranoja. Updates #1096.

[plorkyeran]

(In [3961]) Fix undefined behavior sometimes resulting in a crash when copying styles to or from storage when the style name already exists. Patch by Paranoja. Closes #1096.