mesa: don't crash in KHR_no_error uniform variants when location == -1

mesa: don't crash in KHR_no_error uniform variants when location == -1

From Seciton 7.6 (UNIFORM VARIABLES) of the OpenGL 4.5 spec:

  "If the value of location is -1, the Uniform* commands will
  silently ignore the data passed in, and the current uniform values
  will not be changed.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>