Prevent the stackwalker from wandering beyond the bottom of the stack

[gabrielesvelto]

We sometimes encounter crashes where we wander off the bottom of the stack somehow, here's an example. The last few frames are:

...
26 	XUL 			XRE_main(int, char**, mozilla::BootstrapConfig const&) 	[toolkit/xre/nsAppRunner.cpp:5940] 	cfi
27 	firefox 		do_main(int, char**, char**) 	[browser/app/nsBrowserApp.cpp:227] 	inlined
27 	firefox 		main 	[browser/app/nsBrowserApp.cpp:445] 	cfi
28 	dyld 			start 		cfi
29 	libgkcodecs.dylib 	oc_dec_ac_coeff_unpack 	[media/libtheora/lib/decode.c:1132] 	inlined
29 	libgkcodecs.dylib 	oc_dec_residual_tokens_unpack 	[media/libtheora/lib/decode.c:1194] 	inlined
29 	libgkcodecs.dylib 	th_decode_packetin 	[media/libtheora/lib/decode.c:2823] 	scan
...

Frame 28 should have been the last, but somehow we've found other frames. This might be caused by buggy debug information.