perf symbols: Check for valid dso before creating map

perf symbols: Check for valid dso before creating map

dso__new() can return NULL. Hence verify dso before creating a new map.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anton Arapov <anton@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20120531114656.23691.54223.sendpatchset@srdronam.in.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>