Do not create a kernel cache file if build failed. Also disregard any

existing cache files with size of zero. Should close #164.

src/oclHashcat.c

@@ -13547,7 +13547,7 @@ int main (int argc, char **argv)
 
         struct stat cst;
 
-        if (stat (cached_file, &cst) == -1)
+        if ((stat (cached_file, &cst) == -1) || cst.st_size == 0)
         {
           cached = 0;
         }

src/shared.c

@@ -8671,6 +8671,8 @@ void load_kernel (const char *kernel_file, int num_devices, size_t *kernel_lengt
 }
 
 void writeProgramBin (char *dst, u8 *binary, size_t binary_size)
+{
+  if (binary_size > 0)
   {
     FILE *fp = fopen (dst, "wb");
 
@@ -8679,6 +8681,7 @@ void writeProgramBin (char *dst, u8 *binary, size_t binary_size)
     fflush (fp);
     fclose (fp);
   }
+}
 
 /**
  * restore