glsl: Fix memory error when creating the supported version string.

glsl: Fix memory error when creating the supported version string.

Passing ralloc_vasprintf_append a 0-byte allocation doesn't work.  If
passed a non-NULL argument, ralloc calls strlen to find the end of the
string.  Since there's no terminating '\0', it runs off the end.

Fixes a crash introduced in 14880a510a1a288df0778395097d5a52806abfb0.