Incidentally, fixing intermittent error on srlp and btctx unit tests

This error was happening intermittently in my local, and now has
happened in the CI as well. Turns out ferror was not working well
with fclose, since it would mistakenly report an error while
errno is set to 0.

firmware/src/powhsm/test/btctx/test_btctx.c

@@ -54,12 +54,14 @@ int read_hex_file(const char* file_name, unsigned char** buffer, size_t* len) {
         fread(tmp, 2, 1, f);
         read_hex(tmp, 2, *buffer + off);
     }
-    fclose(f);
-
     if (ferror(f)) {
         return -1;
     }
 
+    if (fclose(f)) {
+        return -1;
+    }
+
     return 0;
 }
 

firmware/src/powhsm/test/srlp/test_srlp.c

@@ -204,12 +204,15 @@ int read_block_file(const char* file_name, char** buffer, size_t* len) {
 
     *buffer = malloc(*len);
     fread(*buffer, *len, 1, f);
-    fclose(f);
 
     if (ferror(f)) {
         return -1;
     }
 
+    if (fclose(f)) {
+        return -1;
+    }
+
     return 0;
 }