mesa: Returns error GL_INVALID_VALUE if num of buffers in glDeleteFramebuffers is negative

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

Fixes 1 dEQP test:
* dEQP-GLES3.functional.negative_api.buffer.delete_framebuffers