edison: fix SPI IO pointers usage

Check if SPI IO pointers are holding valid data before using them.

Signed-off-by: Jianxun Zhang <jianxun.zhang@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Mihai Serban <mihai.serban@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/common.h

@@ -36,6 +36,13 @@
 #define MRAA_MAIN_PLATFORM_OFFSET 0
 #define MRAA_SUB_PLATFORM_OFFSET 1
 
+/** Executes function func and returns its result in case of error
+ */
+#define MRAA_RETURN_FOR_ERROR(func) do { \
+                                      mraa_result_t res; \
+                                      res = func; \
+                                      if (res != MRAA_SUCCESS) \
+                                      return res;} while(0)
 
 /** @file
  *