Fix file_handler process race condition

The current process could be waited both by run_file_handler() and
file_handler_stop().

To avoid the race condition, wait the process without closing, then
close with mutex locked.

app/src/util/process.c

@@ -3,12 +3,12 @@
 #include "log.h"
 
 bool
-process_check_success(process_t proc, const char *name) {
+process_check_success(process_t proc, const char *name, bool close) {
     if (proc == PROCESS_NONE) {
         LOGE("Could not execute \"%s\"", name);
         return false;
     }
-    exit_code_t exit_code = process_wait(proc, true);
+    exit_code_t exit_code = process_wait(proc, close);
     if (exit_code) {
         if (exit_code != NO_EXIT_CODE) {
             LOGE("\"%s\" returned with value %" PRIexitcode, name, exit_code);