mesa: Make sure the buffer exists in _mesa_lookup_bufferobj_err
mesa: Make sure the buffer exists in _mesa_lookup_bufferobj_err

Generate GL_INVALID_OPERATION and return NULL when the buffer object
hasn't been created.  All callers expect this.

v2: Use a more concise error message.

Cc: Laura Ekstrand <94745df4bd94de756ea5436584fec066fc7898d5@jlekstrand.net>
Reviewed-by: Laura Ekstrand <94745df4bd94de756ea5436584fec066fc7898d5@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler