mesa: Fix incorrect assertion in init_teximage_fields_ms

mesa: Fix incorrect assertion in init_teximage_fields_ms

_BaseFormat is a GLenum (unsigned int) so testing if its value is
greater than 0 to detect the cases where _mesa_base_tex_format
returns -1 doesn't work.

Fixing the assertion breaks the arb_texture_view-lifetime-format
piglit test on nouveau, since that test calls
_mesa_base_tex_format with GL_R16F with a context that does not
have ARB_texture_float, so it returns -1 for the BaseFormat, which
was not being caught properly by the ASSERT in init_teximage_fields_ms
until now.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>