library: minor fix for procps_meminfo_select

library: minor fix for procps_meminfo_select

Thus function returns a pointer, not an int, so if there is
an error return NULL and not -errno.

proc/meminfo.c: In function ‘procps_meminfo_select’:
proc/meminfo.c:994:20: warning: return makes pointer from integer
without a cast [-Wint-conversion]
             return -ENOMEM;

References:
 commit 407f1b71de3c4baed23d897b86ad316b64ad1ec9