Fix build (-Werror) on platforms where char is unsigned.

The problem could be easily verified by building with -funsigned-char.

This was developed and tested on OpenBSD with oRTP 0.22.

--- src/b64.c.orig	Sun Dec  8 23:52:27 2013
+++ src/b64.c	Mon Dec  9 00:03:52 2013
@@ -181,7 +181,7 @@ static size_t b64_encode_(  unsigned char const *src
 
         for(; NUM_PLAIN_DATA_BYTES <= srcSize; srcSize -= NUM_PLAIN_DATA_BYTES)
         {
-            char    characters[NUM_ENCODED_DATA_BYTES];
+            unsigned char    characters[NUM_ENCODED_DATA_BYTES];
 
             /* 
              * 
@@ -197,38 +197,38 @@ static size_t b64_encode_(  unsigned char const *src
              */
 
             /* characters[0] is the 6 left-most bits of src[0] */
-            characters[0] = (char)((src[0] & 0xfc) >> 2);
+            characters[0] = ((src[0] & 0xfc) >> 2);
             /* characters[0] is the right-most 2 bits of src[0] and the left-most 4 bits of src[1] */
-            characters[1] = (char)(((src[0] & 0x03) << 4) + ((src[1] & 0xf0) >> 4));
+            characters[1] = (((src[0] & 0x03) << 4) + ((src[1] & 0xf0) >> 4));
             /* characters[0] is the right-most 4 bits of src[1] and the 2 left-most bits of src[2] */
-            characters[2] = (char)(((src[1] & 0x0f) << 2) + ((src[2] & 0xc0) >> 6));
+            characters[2] = (((src[1] & 0x0f) << 2) + ((src[2] & 0xc0) >> 6));
             /* characters[3] is the right-most 6 bits of src[2] */
-            characters[3] = (char)(src[2] & 0x3f);
+            characters[3] = (src[2] & 0x3f);
 
 #ifndef __WATCOMC__
-            assert(characters[0] >= 0 && characters[0] < 64);
-            assert(characters[1] >= 0 && characters[1] < 64);
-            assert(characters[2] >= 0 && characters[2] < 64);
-            assert(characters[3] >= 0 && characters[3] < 64);
+            assert(characters[0] < 64);
+            assert(characters[1] < 64);
+            assert(characters[2] < 64);
+            assert(characters[3] < 64);
 #endif /* __WATCOMC__ */
 
             src += NUM_PLAIN_DATA_BYTES;
-            *p++ = b64_chars[(unsigned char)characters[0]];
+            *p++ = b64_chars[characters[0]];
             assert(NULL != strchr(b64_chars, *(p-1)));
             ++len;
             assert(len != lineLen);
 
-            *p++ = b64_chars[(unsigned char)characters[1]];
+            *p++ = b64_chars[characters[1]];
             assert(NULL != strchr(b64_chars, *(p-1)));
             ++len;
             assert(len != lineLen);
 
-            *p++ = b64_chars[(unsigned char)characters[2]];
+            *p++ = b64_chars[characters[2]];
             assert(NULL != strchr(b64_chars, *(p-1)));
             ++len;
             assert(len != lineLen);
 
-            *p++ = b64_chars[(unsigned char)characters[3]];
+            *p++ = b64_chars[characters[3]];
             assert(NULL != strchr(b64_chars, *(p-1)));
 
             if( ++len == lineLen &&