Bug 1780155 - Remove unnecessary + 1 in StringBuffer ExtractWellSized. r=sfink

This probably goes back to null-terminated strings.

Differential Revision: https://phabricator.services.mozilla.com/D152146

js/src/util/StringBuffer.cpp

@@ -34,7 +34,7 @@ static CharT* ExtractWellSized(Buffer& cb) {
   /* For medium/big buffers, avoid wasting more than 1/4 of the memory. */
   MOZ_ASSERT(capacity >= length);
   if (length > Buffer::sMaxInlineStorage && capacity - length > length / 4) {
-    CharT* tmp = allocPolicy.pod_realloc<CharT>(buf, capacity, length + 1);
+    CharT* tmp = allocPolicy.pod_realloc<CharT>(buf, capacity, length);
     if (!tmp) {
       allocPolicy.free_(buf);
       return nullptr;