Support large value offset in memStore

lib/opFns.js

@@ -842,13 +842,17 @@ function memStore (runState, offset, val, valOffset, length, skipSubMem) {
   length = length.toNumber()
 
   // Limit to the length of `val` (including the offset)
-  var valLength = BN.min(valOffset.addn(length), new BN(val.length))
-
-  // Limit to the length of `val` or the requested length
-  valLength = Math.min(valLength, length)
+  // NOTE: toNumber() is safe here as it will at most as big as val.length
+  var valLength
+  if (valOffset.gten(val.length)) {
+    valLength = 0
+  } else {
+    // NOTE: valOffset should be toNumber safe here
+    valLength = Math.min(val.length - valOffset.toNumber(), length)
+  }
 
   // NOTE: cheat here a bit, if `valOffset` is out of range, just resort to copying zeroes
-  if (valOffset.bitLength() <= 53) {
+  if (valLength > 0 && valOffset.bitLength() <= 53) {
     valOffset = valOffset.toNumber()
 
     // read max possible from the value