Prevent buffer overflow in _al_bvcformata

[connorjclark]

On Windows + ASAN, I got a report of a buffer overflow from this call to al_ustr_newf:

HRESULT _al_create_default_primitives_hlsl_vertex_shader(LPDIRECT3DDEVICE9 device, LPD3DXEFFECT *shader)
{
   ALLEGRO_USTR *full_source = al_ustr_newf("%s\n%s",
   _al_get_default_hlsl_vertex_shader(), technique_source_vertex_v2);
   LPD3DXBUFFER errors;

The cause was a missing null-terminator + usage of strlen in _al_bvcformata. With DEBUGMODE set, this string is grown one byte at a time (which is probably needed to repro). All other usages of exvsnprintf in this file set a null terminator after calling it, except for this one.

Gemini helped me diagnose this: https://gemini.google.com/share/ac2168d6f6ad