babl_model_new(): fix global-buffer-overflow

babl_model_new(): fix global-buffer-overflow

If we pass a string into this function, and this string is shorter than
sizeof(Babl), macro BABL_IS_BABL() will read past string bounds,
and bad things may happen.

NOTE: if a string will be passed into this function, that is not
handled by those  if (!strcmp (arg, "<...>")),  global-buffer-overflow
will still happen. i am not sure if/what can be done about it :(

Fixes following error:
=================================================================
==31464==ERROR: AddressSanitizer: global-buffer-overflow on address 0x7f1c25907940 at pc 0x7f1c258f7636 bp 0x7ffdc4838670 sp 0x7ffdc4838668
READ of size 4 at 0x7f1c25907940 thread T0
    0 0x7f1c258f7635 in babl_model_new /home/lebedevri/src/_GIMP/babl/babl/babl-model.c:114
    1 0x7f1c258e4ce5 in babl_core_init /home/lebedevri/src/_GIMP/babl/babl/babl-core.c:128
    2 0x7f1c258e1379 in babl_init /home/lebedevri/src/_GIMP/babl/babl/babl.c:145
    3 0x7f1c2a0923d1 in gegl_post_parse_hook (/usr/local/lib/libgegl-0.3.so.0+0x523d1)
    4 0x7f1c25195238 in g_option_context_parse (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x55238)
    5 0x7f1c25196193 in g_option_context_parse_strv (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x56193)
    6 0x48b8cf in main (/usr/local/bin/gimp-2.9+0x48b8cf)
    7 0x7f1c241a9b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    8 0x486b68 (/usr/local/bin/gimp-2.9+0x486b68)

0x7f1c25907943 is located 0 bytes to the right of global variable '*.LC1' from 'babl-core.c' (0x7f1c25907940) of size 3
  '*.LC1' is ascii string 'id'
SUMMARY: AddressSanitizer: global-buffer-overflow /home/lebedevri/src/_GIMP/babl/babl/babl-model.c:114 babl_model_new
Shadow bytes around the buggy address:
  0x0fe404b18f10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0fe404b18f20: 05 f9 f9 f9 f9 f9 f9 f9[03]f9 f9 f9 f9 f9 f9 f9
  0x0fe404b18f30: 07 f9 f9 f9 f9 f9 f9 f9 07 f9 f9 f9 f9 f9 f9 f9
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe
==31464==ABORTING