mpm/ac: work around scan-build warnings

mpm/ac: work around scan-build warnings

util-mpm-ac.c:531:32: warning: Result of 'malloc' is converted to a pointer of type 'uint16_t[256]', which is incompatible with sizeof operand type 'uint16_t' [unix.MallocSizeof]
        ctx->state_table_u16 = SCMalloc(ctx->state_count *
                               ^~~~~~~~
./util-mem.h:35:18: note: expanded from macro 'SCMalloc'
 #define SCMalloc malloc
                 ^~~~~~
util-mpm-ac.c:575:32: warning: Result of 'malloc' is converted to a pointer of type 'uint32_t[256]', which is incompatible with sizeof operand type 'uint32_t' [unix.MallocSizeof]
        ctx->state_table_u32 = SCMalloc(ctx->state_count *
                               ^~~~~~~~
./util-mem.h:35:18: note: expanded from macro 'SCMalloc'
 #define SCMalloc malloc
                 ^~~~~~
2 warnings generated.

Bug: #3148.