Require explicit length in s3fs_decode64 (#1755)

This is available from std::string::size in callers.

src/string_util.cpp

@@ -432,16 +432,15 @@ inline unsigned char char_decode64(const char ch)
     return by;
 }
 
-unsigned char* s3fs_decode64(const char* input, size_t* plength)
+unsigned char* s3fs_decode64(const char* input, size_t input_len, size_t* plength)
 {
     unsigned char* result;
-    if(!input || 0 == strlen(input) || !plength){
+    if(!input || 0 == input_len || !plength){
         return NULL;
     }
-    result = new unsigned char[strlen(input) / 4 * 3];
+    result = new unsigned char[input_len / 4 * 3];
 
     unsigned char parts[4];
-    size_t input_len = strlen(input);
     size_t rpos;
     size_t wpos;
     for(rpos = 0, wpos = 0; rpos < input_len; rpos += 4){