lua/flowvarlib: check malloc result

lua/flowvarlib: check malloc result

src/util-lua-flowvarlib.c:110:12: warning: If memory allocation fails, then there is a possible null pointer dereference: buf [nullPointerOutOfMemory]
    memcpy(buf, value, len);
           ^
src/util-lua-flowvarlib.c:109:28: note: Assuming allocation function fails
    uint8_t *buf = SCMalloc(len + 1);
                           ^
src/util-lua-flowvarlib.c:109:28: note: Assignment 'buf=malloc(len+1)', assigned value is 0
    uint8_t *buf = SCMalloc(len + 1);
                           ^
src/util-lua-flowvarlib.c:110:12: note: Null pointer dereference
    memcpy(buf, value, len);
           ^
src/util-lua-flowvarlib.c:111:5: warning: If memory allocation fails, then there is a possible null pointer dereference: buf [nullPointerOutOfMemory]
    buf[len] = '\0';
    ^
src/util-lua-flowvarlib.c:109:28: note: Assuming allocation function fails
    uint8_t *buf = SCMalloc(len + 1);
                           ^
src/util-lua-flowvarlib.c:109:28: note: Assignment 'buf=malloc(len+1)', assigned value is 0
    uint8_t *buf = SCMalloc(len + 1);
                           ^
src/util-lua-flowvarlib.c:111:5: note: Null pointer dereference
    buf[len] = '\0';