+ fix segfault if compress() return empty string

src/server/kiwix-serve.cpp

@@ -17,6 +17,8 @@
  * MA 02110-1301, USA.
  */
 
+#define KIWIX_MIN_CONTENT_SIZE_TO_DEFLATE 100
+
 #ifdef __APPLE__
 #import <sys/types.h>
 #import <sys/sysctl.h>
@@ -344,23 +346,32 @@ static int accessHandlerCallback(void *cls,
   /* Compute the lengh */
   contentLength = content.size();
 
+  /* Should be deflate */
+  bool deflated = 
+    contentLength > KIWIX_MIN_CONTENT_SIZE_TO_DEFLATE &&
+    acceptEncodingDeflate &&
+    mimeType.find("text/") != string::npos; 
+
   /* Compress the content if necessary */
-  if (acceptEncodingDeflate && mimeType.find("text/") != string::npos) {
+  if (deflated) {
     pthread_mutex_lock(&compressorLock);
     comprLen = COMPRESSOR_BUFFER_SIZE;
-
     compress(compr, &comprLen, (const Bytef*)(content.data()), contentLength);
 
+    if (comprLen > 2 && comprLen < contentLength) {
+
       /* /!\ Internet Explorer has a bug with deflate compression.
 	 It can not handle the first two bytes (compression headers)
 	 We need to chunk them off (move the content 2bytes)
 	 It has no incidence on other browsers
 	 See http://www.subbu.org/blog/2008/03/ie7-deflate-or-not and comments */
-    compr++;
-    compr++;
+      compr += 2;
 
       content = string((char *)compr, comprLen);
       contentLength = comprLen;
+    } else {
+      deflated = false;
+    }
 
     pthread_mutex_unlock(&compressorLock);
   }
@@ -377,7 +388,7 @@ static int accessHandlerCallback(void *cls,
     httpResponseCode = MHD_HTTP_FOUND;
   } else {
     /* Add if necessary the content-encoding */
-    if (acceptEncodingDeflate && mimeType.find("text/") != string::npos) {
+    if (deflated) {
       MHD_add_response_header(response, "Content-encoding", "deflate");
     }