Fix: out of bound array access in filter code

Found by Coverity:

*** CID 1372124:  Memory - illegal accesses  (OVERRUN)
/liblttng-ust/lttng-filter.c: 139 in print_op()
133
134     const char *print_op(enum filter_op op)
135     {
136             if (op >= NR_FILTER_OPS)
137                     return "UNKNOWN";
138             else
>>>     CID 1372124:  Memory - illegal accesses  (OVERRUN)
>>>     Overrunning array "opnames" of 74 8-byte elements at element index 78 (byte offset 624) using index "op" (which evaluates to 78).
139                     return opnames[op];
140     }
141
142     static
143     int apply_field_reloc(struct lttng_event *event,

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>