Issue 1734: Streams not flushed if not running actual benchmarks (#1735)

Consistently flush Out and Err streams, otherwise they might not get flushed
and the output lost when using custom streams.

Fixes #1734.

src/benchmark.cc

@@ -577,12 +577,16 @@ size_t RunSpecifiedBenchmarks(BenchmarkReporter* display_reporter,
     Err << "A custom file reporter was provided but "
            "--benchmark_out=<file> was not specified."
         << std::endl;
+    Out.flush();
+    Err.flush();
     std::exit(1);
   }
   if (!fname.empty()) {
     output_file.open(fname);
     if (!output_file.is_open()) {
       Err << "invalid file name: '" << fname << "'" << std::endl;
+      Out.flush();
+      Err.flush();
       std::exit(1);
     }
     if (!file_reporter) {
@@ -597,10 +601,16 @@ size_t RunSpecifiedBenchmarks(BenchmarkReporter* display_reporter,
   }
 
   std::vector<internal::BenchmarkInstance> benchmarks;
-  if (!FindBenchmarksInternal(spec, &benchmarks, &Err)) return 0;
+  if (!FindBenchmarksInternal(spec, &benchmarks, &Err)) {
+    Out.flush();
+    Err.flush();
+    return 0;
+  }
 
   if (benchmarks.empty()) {
     Err << "Failed to match any benchmarks against regex: " << spec << "\n";
+    Out.flush();
+    Err.flush();
     return 0;
   }
 
@@ -611,6 +621,8 @@ size_t RunSpecifiedBenchmarks(BenchmarkReporter* display_reporter,
     internal::RunBenchmarks(benchmarks, display_reporter, file_reporter);
   }
 
+  Out.flush();
+  Err.flush();
   return benchmarks.size();
 }