Throw a few more errors for invalid inputs (#65)

Fixes #39.

jsbi.mjs

@@ -399,6 +399,10 @@ class JSBI extends Array {
   static asIntN(n, x) {
     if (x.length === 0) return x;
     n = Math.floor(n);
+    if (n < 0) {
+      throw new RangeError(
+          'Invalid value: not (convertible to) a safe integer');
+    }
     if (n === 0) return JSBI.__zero();
     // If {x} has less than {n} bits, return it directly.
     if (n >= JSBI.__kMaxLengthBits) return x;
@@ -426,6 +430,10 @@ class JSBI extends Array {
   static asUintN(n, x) {
     if (x.length === 0) return x;
     n = Math.floor(n);
+    if (n < 0) {
+      throw new RangeError(
+          'Invalid value: not (convertible to) a safe integer');
+    }
     if (n === 0) return JSBI.__zero();
     // If {x} is negative, simulate two's complement representation.
     if (x.sign) {
@@ -711,6 +719,7 @@ class JSBI extends Array {
         }
       }
     }
+    if (sign !== 0 && radix !== 10) return null;
     // Skip leading zeros.
     while (current === 0x30) {
       leadingZero = true;
@@ -809,7 +818,6 @@ class JSBI extends Array {
     }
 
     // Get result.
-    if (sign !== 0 && radix !== 10) return null;
     result.sign = (sign === -1);
     return result.__trim();
   }

tests/tests.mjs

@@ -83,7 +83,8 @@ const TESTS = [
 // https://github.com/GoogleChromeLabs/jsbi/issues/36
 (function() {
   const VALID = ['123', ' 123 ', '   123   '];
-  const INVALID = ['x123', 'x 123', ' 123x', '123 x', '123  xx', '123 ?a'];
+  const INVALID = ['x123', 'x 123', ' 123x', '123 x', '123  xx', '123 ?a',
+                   '-0o0', '-0x0', '-0b0', '-0x1'];
   for (const v of VALID) {
     const result = JSBI.BigInt(v);
     console.assert(JSBI.equal(result, JSBI.BigInt(123)));