rocksdb: Fix 'Division by zero' scan-build warning

Summary:
scan-build complains with division by zero warning in a test. Added an assertion to prevent this.

scan-build report: http://home.fburl.com/~sugak/latest6/report-c61be9.html#EndPath

Test Plan:
Make sure scan-build does not report 'Division by zero' and all tests are passing.
```lang=bash
% make analyze
% make check
```

Reviewers: igor, meyering

Reviewed By: meyering

Subscribers: sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33495

util/dynamic_bloom_test.cc

@@ -177,10 +177,10 @@ TEST(DynamicBloomTest, perf) {
         ++count;
       }
     }
+    ASSERT_EQ(count, num_keys);
     elapsed = timer.ElapsedNanos();
     fprintf(stderr, "standard bloom, avg query latency %" PRIu64 "\n",
             elapsed / count);
-    ASSERT_TRUE(count == num_keys);
 
     // Locality enabled version
     DynamicBloom blocked_bloom(&arena, num_keys * 10, 1, num_probes);

util/env_posix.cc

@@ -401,16 +401,16 @@ class PosixMmapFile : public WritableFile {
     return s;
   }
 
-  bool UnmapCurrentRegion() {
-    bool result = true;
+  Status UnmapCurrentRegion() {
     TEST_KILL_RANDOM(rocksdb_kill_odds);
     if (base_ != nullptr) {
       if (last_sync_ < limit_) {
         // Defer syncing this data until next Sync() call, if any
         pending_sync_ = true;
       }
-      if (munmap(base_, limit_ - base_) != 0) {
-        result = false;
+      int munmap_status = munmap(base_, limit_ - base_);
+      if (munmap_status != 0) {
+        return IOError(filename_, munmap_status);
       }
       file_offset_ += limit_ - base_;
       base_ = nullptr;
@@ -423,7 +423,7 @@ class PosixMmapFile : public WritableFile {
         map_size_ *= 2;
       }
     }
-    return result;
+    return Status::OK();
   }
 
   Status MapNewRegion() {
@@ -498,13 +498,15 @@ class PosixMmapFile : public WritableFile {
       assert(dst_ <= limit_);
       size_t avail = limit_ - dst_;
       if (avail == 0) {
-        if (UnmapCurrentRegion()) {
-          Status s = MapNewRegion();
-          if (!s.ok()) {
-            return s;
-          }
-          TEST_KILL_RANDOM(rocksdb_kill_odds);
+        Status s = UnmapCurrentRegion();
+        if (!s.ok()) {
+          return s;
         }
+        s = MapNewRegion();
+        if (!s.ok()) {
+          return s;
+        }
+        TEST_KILL_RANDOM(rocksdb_kill_odds);
       }
 
       size_t n = (left <= avail) ? left : avail;
@@ -524,7 +526,8 @@ class PosixMmapFile : public WritableFile {
 
     TEST_KILL_RANDOM(rocksdb_kill_odds);
 
-    if (!UnmapCurrentRegion()) {
+    s = UnmapCurrentRegion();
+    if (!s.ok()) {
       s = IOError(filename_, errno);
     } else if (unused > 0) {
       // Trim the extra space at the end of the file