mesa: fix more valgrind warnings

mesa: fix more valgrind warnings

==27540== Invalid read of size 4
==27540==    at 0x96277B7: _mesa_make_extension_string (string3.h:144)
==27540==    by 0x9604E78: _mesa_make_current (context.c:1514)
==27540==    by 0x9602A8B: st_api_make_current (st_manager.c:789)
==27540==    by 0x45406E7: ???
==27540==  Address 0xad35b30 is 3,688 bytes inside a block of size 3,691 alloc'd
==27540==    at 0x4025315: calloc (vg_replace_malloc.c:467)
==27540==    by 0x9627641: _mesa_make_extension_string (extensions.c:910)
==27540==    by 0x9604E78: _mesa_make_current (context.c:1514)
==27540==    by 0x9602A8B: st_api_make_current (st_manager.c:789)
==27540==    by 0x45406E7: ???

And:

==28351== Invalid write of size 2
==28351==    at 0x4C087CC: _mesa_make_extension_string (string3.h:144)
==28351==    by 0x4BE6198: _mesa_make_current (context.c:1514)
==28351==    by 0x4BD4CAB: st_api_make_current (st_manager.c:789)
==28351==  Address 0x48dd1f3 is 19 bytes inside a block of size 20 alloc'd
==28351==    at 0x4025315: calloc (vg_replace_malloc.c:467)
==28351==    by 0x4C08711: _mesa_make_extension_string (extensions.c:778)
==28351==    by 0x4BE6198: _mesa_make_current (context.c:1514)
==28351==    by 0x4BD4CAB: st_api_make_current (st_manager.c:789)
==28351==
==28351== Invalid read of size 4
==28351==    at 0x4C087EC: _mesa_make_extension_string (extensions.c:806)
==28351==    by 0x4BE6198: _mesa_make_current (context.c:1514)
==28351==    by 0x4BD4CAB: st_api_make_current (st_manager.c:789)
==28351==  Address 0x48dd1f4 is 0 bytes after a block of size 20 alloc'd
==28351==    at 0x4025315: calloc (vg_replace_malloc.c:467)
==28351==    by 0x4C08711: _mesa_make_extension_string (extensions.c:778)
==28351==    by 0x4BE6198: _mesa_make_current (context.c:1514)
==28351==    by 0x4BD4CAB: st_api_make_current (st_manager.c:789)

The first part adds 2, because ' ' and '\0' may be written at the end
of the buffer.