Corrected nanostack coverity warnings

Author: Mika Leppänen

features/nanostack/mbed-mesh-api/source/NanostackEMACInterface.cpp

@@ -27,7 +27,7 @@ class EMACPhy : public NanostackEthernetPhy {
     EMAC &emac;
     uint8_t mac_addr[6];
     int8_t device_id;
-    phy_device_driver_s phy;
+    phy_device_driver_s phy = {};
 };
 
 // GAH! no handles on the callback. Force a single interface

features/nanostack/mbed-mesh-api/source/NanostackPPPInterface.cpp

@@ -52,7 +52,7 @@ class PPPPhy : public NanostackPPPPhy {
     bool active;
     bool powered_up;
     int8_t device_id;
-    phy_device_driver_s phy;
+    phy_device_driver_s phy = {};
 };
 
 nsapi_error_t Nanostack::PPPInterface::initialize()

features/nanostack/nanostack-hal-mbed-cmsis-rtos/arm_hal_random.c

@@ -34,7 +34,10 @@ uint32_t arm_random_seed_get(void)
     uint32_t result = 0;
 #ifdef MBEDTLS_ENTROPY_HARDWARE_ALT
 #if defined(MBEDTLS_PLATFORM_C)
-    mbedtls_platform_setup(NULL);
+    int ret = mbedtls_platform_setup(NULL);
+    if (ret != 0) {
+        return result;
+    }
 #endif /* MBEDTLS_PLATFORM_C */
     /* Grab a seed from a function we provide for mbedtls */
     size_t len;