glsl/cache: correct asprintf error handling

glsl/cache: correct asprintf error handling

From the manpage of asprintf:

   "If memory allocation wasn't possible, or some other error occurs,
    these functions will return -1, and the contents of strp are
    undefined."

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>