pSrc not initialized in load_png

[charlesdaniels]

Current as of 0729b31. I think this was introduced in 631e60f.

pSrc is initialized here, but never gets updated in the branch of PNG_COLOR_TYPE_RGB, which causes segfaults like this one:

Thread 1 "graph_editor" received signal SIGSEGV, Segmentation fault.
0x00007ffff7b6f857 in ag_surface_get64_at (S=0x555556104170, p=0xff0000 <error: Cannot access memory at address 0xff0000>) at ./inline_surface.h:468
468                     return (*(Uint32 *)p);
(gdb) bt
#0  0x00007ffff7b6f857 in ag_surface_get64_at (S=0x555556104170, p=0xff0000 <error: Cannot access memory at address 0xff0000>) at ./inline_surface.h:468
#1  0x00007ffff7b7a757 in AG_SurfaceExportPNG (S=0x555556104170,
    path=0x555556092370 "/home/cad/src/teaching-learning/libraries/libagar/graph_editor/adsf.png", flags=0) at load_png.c:445
#2  0x0000555555555ba5 in ExportGraph (event=0x7fffffffced0) at main.c:197
#3  0x00007ffff7862fd2 in AG_PostEvent (sp=0x0, rp=0x555555df3b40, evname=0x7ffff7bb04be "file-chosen", fmt=0x7ffff7bb04b8 "%s,%p") at event.c:412
#4  0x00007ffff7b3cdfb in ChooseFile (fd=0x555555df3b40, pwin=0x555555def870) at file_dlg.c:344
#5  0x00007ffff7b3cb12 in CheckAccessAndChoose (fd=0x555555df3b40) at file_dlg.c:463
#6  0x00007ffff7b38f97 in PressedOK (event=0x7fffffffd6a0) at file_dlg.c:540
#7  0x00007ffff7862fd2 in AG_PostEvent (sp=0x0, rp=0x555555e4f520, evname=0x7ffff7bac61c "button-pushed", fmt=0x7ffff7bacfe1 "%i") at event.c:412
#8  0x00007ffff7aff201 in MouseButtonUp (event=0x7fffffffde10) at button.c:213
#9  0x00007ffff7862fd2 in AG_PostEvent (sp=0x0, rp=0x555555e4f520, evname=0x7ffff7b860d3 "mouse-button-up", fmt=0x7ffff7bb3911 "%i(button),%i(x),%i(y)")
    at event.c:412
#10 0x00007ffff7b761b6 in PostMouseButtonUp (win=0x555555def870, wid=0x555555e4f520, x=73, y=330, button=AG_MOUSE_LEFT) at mouse.c:207
#11 0x00007ffff7b761ec in PostMouseButtonUp (win=0x555555def870, wid=0x555555df3b40, x=73, y=330, button=AG_MOUSE_LEFT) at mouse.c:215
#12 0x00007ffff7b760d6 in AG_ProcessMouseButtonUp (win=0x555555def870, x=73, y=330, button=AG_MOUSE_LEFT) at mouse.c:302
#13 0x00007ffff7ad1386 in GLX_ProcessEvent (drvCaller=0x0, dev=0x7fffffffe288) at drv_glx.c:784
#14 0x00007ffff7ad454d in GLX_EventSink (es=0x55555576c7c0, event=0x55555576c7e0) at drv_glx.c:2063
#15 0x00007ffff78694e1 in AG_EventSinkTIMERFD () at event.c:1367
#16 0x00007ffff7869cfb in AG_EventLoop () at event.c:1624
#17 0x0000555555556360 in main (argc=1, argv=0x7fffffffe6b8) at main.c:370

I believe the solution is to replicate the initialization from here. I will try this out and see if I can get it fixed.

Also, I think that the BMP file saving logic is affected similarly. JPEG seems to have some other problem I haven't looked into yet.