Don't switch off disassembly completely if bracktrace was long.

We had a user case when $pc was pointing to .bss,
and gdb's "disassemble" disasm'ed... entire .bss -

Generating backtrace
Backtrace is too big (33561281 bytes), reducing depth to 512
Backtrace is too big (33561281 bytes), reducing depth to 256
Backtrace is too big (33561281 bytes), reducing depth to 128
Backtrace is too big (33561281 bytes), reducing depth to 64
Backtrace is too big (33555685 bytes), reducing depth to 64
Backtrace is too big (33555461 bytes), reducing depth to 64
Backtrace is too big (33555461 bytes), reducing depth to 32
Error: Line 15, column 0: "Thread" header expected
('report_uReport' exited with 1)

This patch swithces to "disassemble $pc-20, $pc+64" which
will prevent such disasters, yet produces at least some disasm.

Related to rhbz#995889.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>