Due to round-off error in floating point calculations never make comparisons based on '=' but on SameValue and an appropriate tolerance value:
uses
Math; // for "SameValue"
const
EPS = 1E-9;
...
a := sin (a * theta/t) / 4;
if SameValue(a, 0.0, EPS) then
Comparisons against zero is almost never a problem. Especially in this case, where you
know that a is zero because 1. the argument is zero, 2. sin(0)=0.
The problem in the OP is, that the superformula is coded far too complicated (power(power(cos(..),n2) + power(sin(..),n3), n1) with unrelated variables). A start would be to breakup the formula and check, whether for small values it can be solved asymptotically. Otherwise the value is even mathematically undefined for the independent limits-to-zero of the variables.
One critical part is
power(abs(sin(m * theta / 4) / b), n3), -1 / n1)
with m=theta=n1=mindouble this tries to compute power(0, -2e323) which obviously overflows. Even if n1 is not that small, power(0, -1/n1) will crash because the power of zero with a negative real exponent is invalid!