Free the previous value before re-strdup'ing, and initialise
remoteMountPoint to NULL in fs_entry_create() so the free() is safe on
the newly-created path

Observed via a dtrace leak snapshot on 15.0-RELEASE-p4:

data leaked = 1983, count = 1683

libc.so.7`malloc+0xa8
snmp_hostres.so.6`fs_tbl_process_statfs_entry+0x276
snmp_hostres.so.6`refresh_storage_tbl+0x502
snmp_hostres.so.6`op_hrStorageTable+0x1a
libbsnmp.so.7`0x2a2b1892ba3d
libbsnmp.so.7`snmp_getbulk+0x37f