Fix memory leak: free CString in helpers.go ValToDatum() initialization (#631)

Author: pskrbasu

helpers.go

@@ -154,7 +154,11 @@ func ValToDatum(val interface{}, cinfo *C.ConversionInfo, buffer C.StringInfo) (
 		}
 	}()
 	// init an empty return result
-	datum := C.fdw_cStringGetDatum(C.CString(""))
+	// Allocate CString, use it, then immediately free to avoid memory leak
+	// Using explicit C.free() instead of defer because this is a hot path
+	emptyStr := C.CString("")
+	datum := C.fdw_cStringGetDatum(emptyStr)
+	C.free(unsafe.Pointer(emptyStr))
 
 	// write value into C buffer
 	if err := valToBuffer(val, cinfo.atttypoid, buffer); err != nil {