Bug 1646171 - Fix vsnprintf complaint in Assertions.cpp r=froydnj

For some reason, our clang-plugin didn't notice this before clang 11.

Differential Revision: https://phabricator.services.mozilla.com/D79901

mfbt/Assertions.cpp

@@ -5,6 +5,7 @@
 
 #include "mozilla/Assertions.h"
 #include "mozilla/Atomics.h"
+#include "mozilla/Sprintf.h"
 
 #include <stdarg.h>
 
@@ -34,8 +35,7 @@ MFBT_API MOZ_COLD MOZ_NEVER_INLINE MOZ_FORMAT_PRINTF(1, 2) const
   }
   va_list aArgs;
   va_start(aArgs, aFormat);
-  int ret =
-      vsnprintf(sPrintfCrashReason, sPrintfCrashReasonSize, aFormat, aArgs);
+  int ret = VsprintfLiteral(sPrintfCrashReason, aFormat, aArgs);
   va_end(aArgs);
   MOZ_RELEASE_ASSERT(
       ret >= 0 && size_t(ret) < sPrintfCrashReasonSize,