autothreshold leak detected in ASAN test

[jaromil]

Running #153 detected this:

{
 "name":"autothreshold",
 "type":"filter",
 "color_model":"rgba8888",
 "num_params":"1"
},
 "parameters":[
  {"name":"Slope","type":"double","explanation":"Slope of sigmoidal transfer"}
 ]
}
Frame 0 processed
Frame 10 processed
Frame 20 processed
Frame 30 processed
Frame 40 processed
Frame 50 processed
Frame 60 processed
Frame 70 processed
Frame 80 processed
Frame 90 processed
Test completed successfully. Plugin: autothreshold
Tested 100 frames with 1 parameters
Input: 640x480 test pattern
Output: 640x480 processed frames

=================================================================
==23976==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 307200 byte(s) in 1 object(s) allocated from:
    #0 0x7fe5770f4c57 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x7fe57772c2d2 in f0r_construct /home/jrml/devel/frei0r/src/filter/autothresh0ld/autothresh0ld.c:71

SUMMARY: AddressSanitizer: 307200 byte(s) leaked in 1 allocation(s).