dnn: Fix dangling pointers returned by GetLayerNames

'GetLayerNames' returns an array of 'char' pointers to cstrings
in a 'vector<string>'; unfortunately, once the vector is out of
scope, the strings are destroyed. 'GetLayerNames' callers are then
left with dangling pointers.

This change fixes the problem by expanding the 'strs' buffer
returned by 'GetLayerNames' and copying the vector's cstrings into it.

dnn.cpp

@@ -111,10 +111,34 @@ void Net_GetUnconnectedOutLayers(Net net, IntVector* res) {
 
 void Net_GetLayerNames(Net net, CStrings* names) {
     std::vector< cv::String > cstrs(net->getLayerNames());
-    const char **strs = new const char*[cstrs.size()];
 
-    for (size_t i = 0; i < cstrs.size(); ++i) {
-        strs[i] = cstrs[i].c_str();
+    size_t totalStrLen = 0;
+    for (cv::String str : cstrs) {
+        totalStrLen += str.size();
+    }
+
+    // Compute 1D buffer size required to store
+    // 2D array of null-terminated strings
+    size_t numStrings = cstrs.size();
+    size_t bufferLen = numStrings * sizeof(char*) + (totalStrLen + numStrings) * sizeof(char);
+
+    const char **strs = (const char**)new char[bufferLen];
+    memset(strs, 0, bufferLen);
+
+    char* it = (char*)(strs + cstrs.size());
+    const char* end = (char*)(strs) + bufferLen;
+
+    for (size_t i = 0; i < numStrings; ++i, ++it) {
+        strs[i] = it;
+        size_t strlen = cstrs[i].size();
+
+        // Avoid buffer overrun
+        if(end < it + strlen + 1) {
+            break;
+        }
+
+        memcpy(it, cstrs[i].c_str(), strlen);
+        it += strlen;
     }
 
     names->length = cstrs.size();