[evm precompiles] do *not* ignore unmarshal return value

Because `unmarshal` is marked as `discardable` we silently ignored the
return value of `unmarshal`, which in some test cases indicated that
the source value does not fit into target bigint

constantine/ethereum_evm_precompiles.nim

@@ -206,7 +206,8 @@ func parseRawUint[Name: static Algebra](
   ## Return false if the integer is larger than the field modulus.
   ## Returns true on success.
   var big {.noInit.}: Fp[Name].getBigInt()
-  big.unmarshal(src, bigEndian)
+  if not big.unmarshal(src, bigEndian):
+    return cttEVM_IntLargerThanModulus # `dst` too small for `src`!
 
   if not bool(big < Fp[Name].getModulus()):
     return cttEVM_IntLargerThanModulus