mesa: fix a valgrind warning

mesa: fix a valgrind warning

==5715== Invalid read of size 4
==5715==    at 0x4AA590B: _mesa_make_extension_string (extensions.c:908)
==5715==    by 0x4A83198: _mesa_make_current (context.c:1514)
==5715==    by 0x4A71CAB: st_api_make_current (st_manager.c:789)
==5715==  Address 0x4795730 is 0 bytes inside a block of size 1 alloc'd
==5715==    at 0x4025315: calloc (vg_replace_malloc.c:467)
==5715==    by 0x4AA5B4C: _mesa_make_extension_string (extensions.c:772)
==5715==    by 0x4A83198: _mesa_make_current (context.c:1514)
==5715==    by 0x4A71CAB: st_api_make_current (st_manager.c:789)