buffer: combine checking range of sourceStart in buf.copy

Merging 2 checking range of sourceStart into 1. Plus,
add test case to increase coverage if sourceStart is greater
than length of source.

PR-URL: nodejs#47758
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/buffer.js

@@ -231,8 +231,8 @@ function _copy(source, target, targetStart, sourceStart, sourceEnd) {
     sourceStart = 0;
   } else {
     sourceStart = toInteger(sourceStart, 0);
-    if (sourceStart < 0)
-      throw new ERR_OUT_OF_RANGE('sourceStart', '>= 0', sourceStart);
+    if (sourceStart < 0 || sourceStart > source.length)
+      throw new ERR_OUT_OF_RANGE('sourceStart', `>= 0 && <= ${source.length}`, sourceStart);
   }
 
   if (sourceEnd === undefined) {
@@ -246,12 +246,6 @@ function _copy(source, target, targetStart, sourceStart, sourceEnd) {
   if (targetStart >= target.length || sourceStart >= sourceEnd)
     return 0;
 
-  if (sourceStart > source.length) {
-    throw new ERR_OUT_OF_RANGE('sourceStart',
-                               `<= ${source.length}`,
-                               sourceStart);
-  }
-
   return _copyActual(source, target, targetStart, sourceStart, sourceEnd);
 }
 

test/parallel/test-buffer-alloc.js

@@ -124,7 +124,7 @@ b.copy(Buffer.alloc(0), 1, 1, 1);
 b.copy(Buffer.alloc(1), 1, 1, 1);
 
 // Try to copy 0 bytes from past the end of the source buffer
-b.copy(Buffer.alloc(1), 0, 2048, 2048);
+b.copy(Buffer.alloc(1), 0, 1024, 1024);
 
 // Testing for smart defaults and ability to pass string values as offset
 {

test/parallel/test-buffer-copy.js

@@ -155,8 +155,15 @@ assert.throws(
   {
     code: 'ERR_OUT_OF_RANGE',
     name: 'RangeError',
-    message: 'The value of "sourceStart" is out of range. ' +
-             'It must be >= 0. Received -1'
+  }
+);
+
+// Copy throws if sourceStart is greater than length of source
+assert.throws(
+  () => Buffer.allocUnsafe(5).copy(Buffer.allocUnsafe(5), 0, 100),
+  {
+    code: 'ERR_OUT_OF_RANGE',
+    name: 'RangeError',
   }
 );