Fix Coverity issue: Initialize FlashIAP non-static member in constructor

Author: hugueskamba

drivers/source/FlashIAP.cpp

@@ -46,7 +46,7 @@ static inline bool is_aligned(uint32_t number, uint32_t alignment)
     }
 }
 
-FlashIAP::FlashIAP() : _page_buf(nullptr)
+FlashIAP::FlashIAP() : _flash(), _page_buf(nullptr)
 {
 
 }