Use short instead of char for the decoding[] table.
Also fix an off-by-one error when checking the upper boundary for value_in.

--- a/src/osgDB/ConvertBase64.cpp	2015-07-23 13:14:00.000000000 +0200
+++ b/src/osgDB/ConvertBase64.cpp	2016-12-06 14:05:50.435406287 +0100
@@ -28,10 +28,10 @@
 
     int base64_decode_value(char value_in)
     {
-        static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
-        static const char decoding_size = sizeof(decoding);
+        static const short decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
+        static const int decoding_size = sizeof(decoding) / sizeof(decoding[0]);
         value_in -= 43;
-        if (value_in < 0 || value_in > decoding_size) return -1;
+        if (value_in < 0 || value_in >= decoding_size) return -1;
         return decoding[(int)value_in];
     }
 
--- a/src/osgPlugins/osgjs/Base64.cpp	2015-03-09 12:27:26.000000000 +0100
+++ b/src/osgPlugins/osgjs/Base64.cpp	2016-12-06 14:27:36.284022097 +0100
@@ -20,22 +20,22 @@
 
   const char _from_table[128] =
   {
-    -1, -1, -1, -1, -1, -1, -1, -1, // 0
-    -1, -1, -1, -1, -1, -1, -1, -1, // 8
-    -1, -1, -1, -1, -1, -1, -1, -1, // 16
-    -1, -1, -1, -1, -1, -1, -1, -1, // 24
-    -1, -1, -1, -1, -1, -1, -1, -1, // 32
-    -1, -1, -1, 62, -1, -1, -1, 63, // 40
-    52, 53, 54, 55, 56, 57, 58, 59, // 48
-    60, 61, -1, -1, -1,  0, -1, -1, // 56
-    -1,  0,  1,  2,  3,  4,  5,  6, // 64
-     7,  8,  9, 10, 11, 12, 13, 14, // 72
-    15, 16, 17, 18, 19, 20, 21, 22, // 80
-    23, 24, 25, -1, -1, -1, -1, -1, // 88
-    -1, 26, 27, 28, 29, 30, 31, 32, // 96
-    33, 34, 35, 36, 37, 38, 39, 40, // 104
-    41, 42, 43, 44, 45, 46, 47, 48, // 112
-    49, 50, 51, -1, -1, -1, -1, -1  // 120
+   '\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff', // 0
+   '\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff', // 8
+   '\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff', // 16
+   '\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff', // 24
+   '\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff', // 32
+   '\xff','\xff','\xff',    62,'\xff','\xff','\xff',    63, // 40
+       52,    53,    54,    55,    56,    57,    58,    59, // 48
+       60,    61,'\xff','\xff','\xff',     0,'\xff','\xff', // 56
+   '\xff',     0,     1,     2,     3,     4,     5,     6, // 64
+        7,     8,     9,    10,    11,    12,    13,    14, // 72
+       15,    16,    17,    18,    19,    20,    21,    22, // 80
+       23,    24,    25,'\xff','\xff','\xff','\xff','\xff', // 88
+   '\xff',    26,    27,    28,    29,    30,    31,    32, // 96
+       33,    34,    35,    36,    37,    38,    39,    40, // 104
+       41,    42,    43,    44,    45,    46,    47,    48, // 112
+       49,    50,    51,'\xff','\xff','\xff','\xff','\xff'  // 120
   };
   const char* from_table = _from_table;
 }