Minor assertion verifying that no variable has an "=" in its values.

[PanoLeRamix]

This is a minor assertion verifying that no variable has an "=" in its values.
It changes almost nothing, but might save to others the time it took to me to understand the problem while I was using NPF giving it values as "CFLAGS=-DNAIVE=1", making it constantly crash.
For example, this:

%variables
PARAM=[1-3]

would be fully valid, as this:

%variables
PARAM={a=1,a=2,a=3}

would throw an exception.

[tbarbette]

Thanks!