Roman Lebedev authored Jun 24, 2015 and Mel Henning committed Jun 29, 2015

If one of tests fail and images are different, we write image
difference under a new name. We use very scary looking memcpy()
to replace last 4 symbols of filename with "-diff.png", but
since sizeof("-diff.png") == 10, we were reading 1 byte past
string bound.

Fixes AddressSanitiser complaint like:
...
PASS apply-lens3.xml (OpenCL)
/home/lebedevri/src/_GIMP/gegl/tests/compositions/reference/bump-map.png and /home/lebedevri/src/_GIMP/gegl/tests/compositions/output/bump-map.png differ
  wrong pixels   : 46/150000 (0.03%)
  max ?e         : 0.214
  avg ?e (wrong) : 0.111(wrong) 0.000(total)
=================================================================
==29799==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000402aa0 at pc 0x4020b0 bp 0x7fffa5051640 sp 0x7fffa5051638
READ of size 11 at 0x000000402aa0 thread T0
    0 0x4020af in main /home/lebedevri/src/_GIMP/gegl/tools/gegl-imgcmp.c:148
    1 0x7fe64b509b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    2 0x40154b (/home/lebedevri/src/_GIMP/gegl/tools/.libs/gegl-imgcmp+0x40154b)

0x000000402aa0 is located 4205152 bytes insideASAN:SIGSEGV
==29799==AddressSanitizer: while reporting a bug found another one.Ignoring.
FAIL bump-map.xml
...