extensions: fast-float: add destroy() callback, fix 2 leaks

extensions: fast-float: add destroy() callback, fix 2 leaks

Fixes these 2 not-so-informative leaks, every test showed them:

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==27868==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 540276 byte(s) in 1 object(s) allocated from:
    0 0x4cf190 in calloc (/home/lebedevri/src/_GIMP/babl/tests/.libs/nop+0x4cf190)
    1 0x7f9cdd208cbb  (<unknown module>)
    2 0x7f9cdd207cca  (<unknown module>)
    3 0x7f9ce06820d8 in babl_extension_load /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:221:7
    4 0x7f9ce06819d6 in babl_extension_load_dir /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:263:19
    5 0x7f9ce06813fa in babl_extension_load_dir_list /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:333:17
    6 0x7f9ce0678c84 in babl_init /home/lebedevri/src/_GIMP/babl/babl/babl.c:151:7
    7 0x50504a in main /home/lebedevri/src/_GIMP/babl/tests/nop.c:26:3
    8 0x7f9cdf64e72f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2072f)

Direct leak of 540276 byte(s) in 1 object(s) allocated from:
    0 0x4cf190 in calloc (/home/lebedevri/src/_GIMP/babl/tests/.libs/nop+0x4cf190)
    1 0x7f9cdd208cbb  (<unknown module>)
    2 0x7f9cdd207cf6  (<unknown module>)
    3 0x7f9ce06820d8 in babl_extension_load /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:221:7
    4 0x7f9ce06819d6 in babl_extension_load_dir /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:263:19
    5 0x7f9ce06813fa in babl_extension_load_dir_list /home/lebedevri/src/_GIMP/babl/babl/babl-extension.c:333:17
    6 0x7f9ce0678c84 in babl_init /home/lebedevri/src/_GIMP/babl/babl/babl.c:151:7
    #7 0x50504a in main /home/lebedevri/src/_GIMP/babl/tests/nop.c:26:3
    8 0x7f9cdf64e72f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2072f)

SUMMARY: AddressSanitizer: 1080552 byte(s) leaked in 2 allocation(s).