heap buffer overflow in filter curves

[jaromil]

{
"name":"Curves",
"type":"filter",
"color_model":"rgba8888",
"num_params":"16"
}=================================================================
==4639==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000ad34 at pc 0x7fd451f7531f bp 0x7ffcbd128650 sp 0x7ffcbd128648
READ of size 4 at 0x60200000ad34 thread T0
#0 0x7fd451f7531e in f0r_update /home/jrml/frei0r/build/../src/filter/curves/curves.c:958:14
#1 0x4ce10b in main /home/jrml/frei0r/test/frei0r-test.c:148:2
#2 0x7fd468bb9d09 in __libc_start_main csu/../csu/libc-start.c:308:16
#3 0x420819 in _start (/home/jrml/frei0r/test/frei0r-test+0x420819)

0x60200000ad34 is located 0 bytes to the right of 4-byte region [0x60200000ad30,0x60200000ad34)
allocated by thread T0 here:
#0 0x49a88d in malloc (/home/jrml/frei0r/test/frei0r-test+0x49a88d)
#1 0x7fd451f6ae38 in f0r_construct /home/jrml/frei0r/build/../src/filter/curves/curves.c:260:20
#2 0x4cdb61 in main /home/jrml/frei0r/test/frei0r-test.c:126:14
#3 0x7fd468bb9d09 in __libc_start_main csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/jrml/frei0r/build/../src/filter/curves/curves.c:958:14 in f0r_update

[Merlimau]

Probably related to this commit: 04ac1c5