perf annotate: Don't throw error for zero length symbols

'perf report --tui' exits with error when it finds a sample of zero
length symbol (i.e. addr == sym->start == sym->end). Actually these are
valid samples. Don't exit TUI and show report with such symbols.

Reported-and-Tested-by: Anton Blanchard <anton@samba.org>
Link: https://lkml.org/lkml/2016/10/8/189
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Chris Riyder <chris.ryder@arm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@kernel.org # v4.9+
Link: http://lkml.kernel.org/r/1479804050-5028-1-git-send-email-ravi.bangoria@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>