Hello Keith,
I suffer from the same problem - crash on startup.
The backtrace is identical (attaching).
The circumstances are the same - I have four monitors with two nvidia cards
using nouveau driver.
I am using xinerama.

After applying your patch the backtrace is different:

double free or corruption (!prev)
(EE)
(EE) Backtrace:
(EE) 0: /usr/lib/xorg/Xorg (xorg_backtrace+0x4a) [0x5637ad9347da]
(EE) 1: /usr/lib/xorg/Xorg (0x5637ad780000+0x1b8489) [0x5637ad938489]
(EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7efc039f4000+0x11f50)
[0x7efc03a05f50]
(EE) 3: /lib/x86_64-linux-gnu/libc.so.6 (gsignal+0x10b) [0x7efc0366ee7b]
(EE) 4: /lib/x86_64-linux-gnu/libc.so.6 (abort+0x151) [0x7efc03670231]
(EE) 5: /lib/x86_64-linux-gnu/libc.so.6 (0x7efc0363a000+0x777b7)
[0x7efc036b17b7]
(EE) 6: /lib/x86_64-linux-gnu/libc.so.6 (0x7efc0363a000+0x7dd5a)
[0x7efc036b7d5a]
(EE) 7: /lib/x86_64-linux-gnu/libc.so.6 (0x7efc0363a000+0x7f81c)
[0x7efc036b981c]
(EE) 8: /usr/lib/xorg/Xorg (PanoramiXCreateConnectionBlock+0x20a)
[0x5637ad87f98a]
(EE) 9: /usr/lib/xorg/Xorg (0x5637ad780000+0x609e1) [0x5637ad7e09e1]
(EE) 10: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xe7)
[0x7efc0365ba87]
(EE) 11: /usr/lib/xorg/Xorg (_start+0x2a) [0x5637ad7ca6ca]

Attaching a full log.

regards,
manio