bootchart: fix two unitialized memory frees

bootchart: fix two unitialized memory frees

The new gcc isn't bad!

In file included from src/bootchart/svg.c:36:0:
src/bootchart/svg.c: In function 'svg_ps_bars':
./src/shared/util.h:524:13: warning: 'enc_name' may be used uninitialized in this function [-Wmaybe-uninitialized]
         free(*(void**) p);
             ^
src/bootchart/svg.c:821:37: note: 'enc_name' was declared here
                 char _cleanup_free_*enc_name;
                                     ^
  CC     src/udev/mtd_probe/mtd_probe-probe_smartmedia.o
  XSLT     man/systemd.unit.5
In file included from src/bootchart/svg.c:36:0:
src/bootchart/svg.c: In function 'svg_pss_graph':
./src/shared/util.h:524:13: warning: 'enc_name' may be used uninitialized in this function [-Wmaybe-uninitialized]
         free(*(void**) p);
             ^
src/bootchart/svg.c:395:37: note: 'enc_name' was declared here
                 char _cleanup_free_*enc_name;
                                     ^