Bug 1083373 part 2: Assert that array index is in-bounds, and cast it from char to size_t to fix clang warning. r=edwin

media/gmp-clearkey/0.1/ClearKeyUtils.cpp

@@ -10,6 +10,8 @@
 #include <vector>
 
 #include "ClearKeyUtils.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
 #include "mozilla/Endian.h"
 #include "mozilla/NullPtr.h"
 #include "openaes/oaes_lib.h"
@@ -106,7 +108,12 @@ EncodeBase64Web(vector<uint8_t> aBinary, string& aEncoded)
     out[i] += (*data >> (shift + 2)) & sMask;
     shift = (shift + 2) % 8;
 
-    out[i] = sAlphabet[out[i]];
+    auto idx = out[i];
+    MOZ_ASSERT(idx >= 0 &&
+               static_cast<size_t>(idx) < MOZ_ARRAY_LENGTH(sAlphabet),
+               "out of bounds index for 'sAlphabet'");
+    // Cast idx to size_t to pacify clang 'Wchar-subscripts' warning:
+    out[i] = sAlphabet[static_cast<size_t>(idx)];
   }
 
   return true;