Correctly test directory on Linux 3.13

Previously the fopen write mode caused the call to spuriously fail.
Also plug file descriptor leak and more carefully check return values.

Author: gaul

README.md

@@ -46,6 +46,7 @@ This section reports those models where mbpfan was tested successfully. It does
 - MacBook Pro 6,2 15"  (Intel i7 - Linux 3.5.0)
 - MacBook Pro 6,2 15"  (Intel i7 - Linux 3.2.0)
 - MacBook Pro 2,2 15"  (Intel Core 2 Duo - Linux 3.4.4)
+- MacBook Air 6,1 13"  (Intel i7 - Linux 3.13)
 - MacBook Air 5,2 13"  (Intel i5 - Linux 3.16)
 - MacBook Air 1,1 13"  (Intel Core Duo - Linux 4.4, Linux 4.8)
 - MacBook Air 7,2 13"  (Intel Core Duo - Linux 4.10)

src/mbpfan.c

@@ -77,13 +77,12 @@ bool is_legacy_sensors_path()
 
 
     // thanks http://stackoverflow.com/questions/18192998/plain-c-opening-a-directory-with-fopen
-    fopen("/sys/devices/platform/coretemp.0/hwmon", "wb");
-
-    if (errno == EISDIR) {
-        return 0;
-    } else {
-        return 1;
+    FILE *file = fopen("/sys/devices/platform/coretemp.0/hwmon", "rb");
+    int isdir = file == NULL && errno == EISDIR;
+    if (file != NULL) {
+        fclose(file);
     }
+    return !isdir;
 
     // 
     // str_kernel_version = strtok(NULL, ".");
@@ -143,9 +142,13 @@ t_sensors *retrieve_sensors()
             sprintf(hwmon_path, "%s%d", path_begin, counter);
 
             // thanks http://stackoverflow.com/questions/18192998/plain-c-opening-a-directory-with-fopen
-            fopen(hwmon_path, "wb");
+            FILE *file = fopen(hwmon_path, "rb");
+            int isdir = file == NULL && errno == EISDIR;
+            if (file != NULL) {
+                fclose(file);
+            }
 
-            if (errno == EISDIR) {
+            if (isdir) {
 
                 path_begin = (char*) malloc(sizeof( char ) * (strlen(hwmon_path) + strlen("/temp") + 1));
                 strcpy(path_begin, hwmon_path);