Make getAdjustedExponentLength in modexp precompile more robust toward > 53 bit number handling

Author: holgerd77

lib/precompiled/05-modexp.js

@@ -25,9 +25,14 @@ function multComplexity (x) {
 }
 
 function getAdjustedExponentLength (data) {
-  var baseLen = new BN(data.slice(0, 32)).toNumber()
+  var expBytesStart
+  try {
+    var baseLen = new BN(data.slice(0, 32)).toNumber()
+    expBytesStart = 96 + baseLen // 96 for base length, then exponent length, and modulus length, then baseLen for the base data, then exponent bytes start
+  } catch (e) {
+    expBytesStart = Number.MAX_SAFE_INTEGER - 32
+  }
   var expLen = new BN(data.slice(32, 64))
-  var expBytesStart = 96 + baseLen // 96 for base length, then exponent length, and modulus length, then baseLen for the base data, then exponent bytes start
   var firstExpBytes = Buffer.from(data.slice(expBytesStart, expBytesStart + 32)) // first word of the exponent data
   firstExpBytes = utils.setLengthRight(firstExpBytes, 32) // reading past the data reads virtual zeros
   firstExpBytes = new BN(firstExpBytes)