Fix build failure in symfile.c::unmap_overlay_command (GCC5 bug)

Fix build failure in symfile.c::unmap_overlay_command (GCC5 bug)

Compilation of (GDB) 7.9.50.20150127-cvs with (GCC) 5.0.0 20150127
fails with

In file included from symfile.c:32:0:
symfile.c: In function 'unmap_overlay_command':
objfiles.h:628:3: error: 'sec' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
   for (osect = objfile->sections; osect < objfile->sections_end; osect++) \
   ^
symfile.c:3442:23: note: 'sec' was declared here
   struct obj_section *sec;
                       ^
cc1: all warnings being treated as errors
make[2]: *** [symfile.o] Error 1
make[2]: Leaving directory `gdb/gdb'

While the bug was reported to GCC as
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64823>,
the attached patch simply initializes sec with NULL.

gdb/ChangeLog:

        * symfile.c (unmap_overlay_command): Initialize sec to NULL.

Tested on x86_64-linux.