libproc-ng & ps: fix warning reported by smatch

libproc-ng & ps: fix warning reported by smatch

proc/ksym.c +279 read_file(54) info: redundant null check on buf calling free()
proc/ksym.c +330 parse_ksyms(41) info: redundant null check on ksyms_data calling free()
proc/ksym.c +332 parse_ksyms(43) info: redundant null check on ksyms_index calling free()
proc/ksym.c +451 sysmap_mmap(110) info: redundant null check on sysmap_index calling free()
proc/procps.h:74:5: warning: undefined preprocessor identifier 'SHARED'
proc/slab.c +145 parse_slabinfo20(24) error: potential null derefence 'prev'.
proc/slab.c +222 parse_slabinfo11(21) error: potential null derefence 'prev'.
ps/stacktrace.c +131 debug(6) error: snprintf() is printing too much 99 vs 16

Signed-off-by: Sami Kerola <kerolasa@iki.fi>