fix unchecked calloc return on Windows

Author: fredroy

Sofa/framework/Helper/src/sofa/helper/BackTrace.cpp

@@ -117,16 +117,19 @@ BackTrace::StackTrace BackTrace::getTrace(size_t maxEntries)
 
     frames = CaptureStackBackTrace(0, 100, stack, nullptr);
     symbol = (SYMBOL_INFO *)calloc(sizeof(SYMBOL_INFO) + 256 * sizeof(char), 1);
-    symbol->MaxNameLen = 255;
-    symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
-
-    for (i = 0; i < frames; i++)
+    if (symbol != nullptr)
     {
-        SymFromAddr(process, (DWORD64)(stack[i]), 0, symbol);
-        result.push_back(symbol->Name);
-    }
+        symbol->MaxNameLen = 255;
+        symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
+
+        for (i = 0; i < frames; i++)
+        {
+            SymFromAddr(process, (DWORD64)(stack[i]), 0, symbol);
+            result.push_back(symbol->Name);
+        }
 
-    free(symbol);
+        free(symbol);
+    }
 #endif
 
     return result;