babl_format_new(): fix global-buffer-overflow

babl_format_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:
=================================================================
==1657==ERROR: AddressSanitizer: global-buffer-overflow on address 0x7fd3026d6c20 at pc 0x7fd3026b6e4a bp 0x7fffaac308a0 sp 0x7fffaac30898
READ of size 4 at 0x7fd3026d6c20 thread T0
    0 0x7fd3026b6e49 in babl_format_new /home/lebedevri/src/_GIMP/babl/babl/babl-format.c:317
    1 0x7fd3026bf44f in construct_double_format /home/lebedevri/src/_GIMP/babl/babl/babl-model.c:259
    2 0x7fd3026bfd12 in babl_model_new /home/lebedevri/src/_GIMP/babl/babl/babl-model.c:204
    3 0x7fd3026acce5 in babl_core_init /home/lebedevri/src/_GIMP/babl/babl/babl-core.c:128
    4 0x7fd3026a9379 in babl_init /home/lebedevri/src/_GIMP/babl/babl/babl.c:145
    5 0x7fd306e5a3d1 in gegl_post_parse_hook (/usr/local/lib/libgegl-0.3.so.0+0x523d1)
    6 0x7fd301f5d238 in g_option_context_parse (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x55238)
    7 0x7fd301f5e193 in g_option_context_parse_strv (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x56193)
    8 0x48b8cf in main (/usr/local/bin/gimp-2.9+0x48b8cf)
    9 0x7fd300f71b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    10 0x486b68 (/usr/local/bin/gimp-2.9+0x486b68)

0x7fd3026d6c23 is located 0 bytes to the right of global variable '*.LC8' from 'babl-model.c' (0x7fd3026d6c20) of size 3
  '*.LC8' is ascii string 'id'
SUMMARY: AddressSanitizer: global-buffer-overflow /home/lebedevri/src/_GIMP/babl/babl/babl-format.c:317 babl_format_new
Shadow bytes around the buggy address:
  0x0ffae04d2d70: f9 f9 f9 f9 00 00 04 f9 f9 f9 f9 f9 00 00 02 f9
=>0x0ffae04d2d80: f9 f9 f9 f9[03]f9 f9 f9 f9 f9 f9 f9 00 02 f9 f9
  0x0ffae04d2d90: f9 f9 f9 f9 05 f9 f9 f9 f9 f9 f9 f9 00 00 00 00
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
==1657==ABORTING