lavu/base64: return meaningful error code.

libavutil/base64.c

@@ -125,7 +125,7 @@ out2:
     *dst++ = v >> 4;
 out1:
 out0:
-    return bits & 1 ? -1 : dst - out;
+    return bits & 1 ? AVERROR_INVALIDDATA : dst - out;
 }
 
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