Update helm.cpp

fix bug allowing integer iat to be used before being set.  two lines of code were swapped to fix the problem.

src/eos/private/helm.cpp

@@ -355,8 +355,8 @@ namespace serif::eos::helmholtz {
     // hash the table location in t, rho:
     jat = (log10(T) - tlo)*tstpi;
     jat = max(0, min(jat, table.jmax-1)); // bounds
-    iat = max(0, min(iat, table.imax-1)); // bounds
     iat = (log10(din)-dlo)*dstpi;
+    iat = max(0, min(iat, table.imax-1)); // bounds
 
     //using the indices, access the table:
     fi[0]  = table.f[iat][jat];