Fix a crash in FormatStringIntoVector

It was wrong to use vararg after vsnprintf() affected it.
Luckily, we don't need to call vsnprintf() on vararg,
since the previous call of vsnprintf() on varargCopy
already gives us the length that we need.

Bug: angleproject:5131
Change-Id: Ie9b62e92ef8ab7e06b51e034c99a5fde20c1ceaf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2453930
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>

Author: y-novikov

src/common/angleutils.cpp

@@ -49,26 +49,26 @@ std::string ArrayIndexString(const std::vector<unsigned int> &indices)
 
 size_t FormatStringIntoVector(const char *fmt, va_list vararg, std::vector<char> &outBuffer)
 {
-    // The state of the va_list passed to vsnprintf is undefined after the call, do a copy in case
-    // we need to grow the buffer.
+    // The state of the va_list passed to vsnprintf is undefined after the call,
+    // do a copy since we don't want to interfere with vararg usage inside the caller.
     va_list varargCopy;
     va_copy(varargCopy, vararg);
 
     // Attempt to just print to the current buffer
     int len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, varargCopy);
     va_end(varargCopy);
+    ASSERT(len >= 0);
 
-    if (len < 0 || static_cast<size_t>(len) >= outBuffer.size())
+    if (static_cast<size_t>(len) >= outBuffer.size())
     {
-        // Buffer was not large enough, calculate the required size and resize the buffer
-        len = vsnprintf(nullptr, 0, fmt, vararg);
+        // Buffer was not large enough, resize it
         outBuffer.resize(len + 1);
 
         // Print again
         va_copy(varargCopy, vararg);
         len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, varargCopy);
         va_end(varargCopy);
+        ASSERT(len >= 0);
     }
-    ASSERT(len >= 0);
     return static_cast<size_t>(len);
 }