Fix crashing with some versions of SDL

[Low-power]

In SDL version 2.0.5 as provided in Debian 9, passing nullptr as the first argument to SDL_GetPrefPath(3) would crash the program:

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Reading symbols from bin/SpaceCadetPinball...done.
(gdb) r
Starting program: bin/SpaceCadetPinball 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffee89b700 (LWP 1038330)]
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Using SDL renderer: opengl

Thread 1 "SpaceCadetPinba" received signal SIGSEGV, Segmentation fault.
strlen () at ../sysdeps/x86_64/strlen.S:106
106	../sysdeps/x86_64/strlen.S: No such file or directory.
(gdb) bt
#0  strlen () at ../sysdeps/x86_64/strlen.S:106
#1  0x00007ffff777172f in ?? () from /usr/lib/x86_64-linux-gnu/libSDL2-2.0.so.0
#2  0x00005555555d63be in winmain::WinMain (lpCmdLine=0x7fffffffe490 "")
    at SpaceCadetPinball/winmain.cpp:112
#3  0x00005555555c7bdd in MainActual (lpCmdLine=0x7fffffffe490 "")
    at SpaceCadetPinball/SpaceCadetPinball.cpp:14
#4  0x00005555555b4333 in main (argc=1, argv=0x7fffffffe5a8)
    at SpaceCadetPinball/SpaceCadetPinball.cpp:26
(gdb) frame 2
#2  0x00005555555d63be in winmain::WinMain (lpCmdLine=0x7fffffffe490 "")
    at SpaceCadetPinball/winmain.cpp:112
112		auto prefPath = SDL_GetPrefPath(nullptr, "SpaceCadetPinball");

I think it is worth to fix as the game fully worked after applying #162 and this patch (guessing #163).

[k4zmu2a]

Suggested change

	auto prefPath = SDL_GetPrefPath("Microsoft", "SpaceCadetPinball");
	auto prefPath = SDL_GetPrefPath("", "SpaceCadetPinball");

SDL_GetPrefPath has a bug in older SDL versions.
Use "" to keep old path and avoid orphaning existing user data.

[Low-power]

Didn't came up with empty string at first, since the man page mentioned neither null pointer nor empty string condition for that argument.

[k4zmu2a]

Merged.
SDL did not update their docs after patching SDL_GetPrefPath.
Newer versions replace null with “”. They do not add double slash when org is “”.
Older versions produce path with double slash, but that should be fine.