mesa: Returns a GL_INVALID_VALUE error if num of texs in glDeleteTextures is negative

Per GLES3 manual for glDeleteTextures
<https://www.khronos.org/opengles/sdk/docs/man3/html/glDeleteTextures.xhtml>,
GL_INVALID_VALUE is generated if n is negative.

Fixes 1 dEQP test:
* dEQP-GLES3.functional.negative_api.texture.deletetextures

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>