Fix #154.

Cache eviction was trying to evict the records until the new blob fits w/o checking if any records are actually cached.

This could happen when a record does not fit into cache:

```
dvl@dvl-mbp cpe2cve % echo "cpe:/a:foo:bar:*" | go run . -cpe 1 -e 1 -cve 1 -cache_size 1 ~/feeds/cve_20200424000000_nvd.json.gz
panic: attempted to evict non-existent record

goroutine 14 [running]:
github.com/facebookincubator/nvdtools/cvefeed.(*Cache).evict(0xc05df0b9f0, 0x0)
        /Users/dvl/go/src/github.com/facebookincubator/nvdtools/cvefeed/cvecache.go:238 +0x140
github.com/facebookincubator/nvdtools/cvefeed.(*Cache).Get(0xc05df0b9f0, 0xc00008e008, 0x1, 0x1, 0x1, 0xc07bb2bd70, 0x10)
        /Users/dvl/go/src/github.com/facebookincubator/nvdtools/cvefeed/cvecache.go:171 +0x425
main.processAll(0xc07bb45b60, 0xc07bb45bc0, 0xc05e3e20f0, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /Users/dvl/go/src/github.com/facebookincubator/nvdtools/cmd/cpe2cve/cpe2cve.go:73 +0xd6a
main.processInput.func1(0xc07bb45b60, 0xc07bb45bc0, 0xc05e3e20f0, 0xc0001340f0, 0xc07bb2bd38, 0xc07bb2bd40)
        /Users/dvl/go/src/github.com/facebookincubator/nvdtools/cmd/cpe2cve/cpe2cve.go:139 +0xcc
created by main.processInput
        /Users/dvl/go/src/github.com/facebookincubator/nvdtools/cmd/cpe2cve/cpe2cve.go:138 +0x309
exit status 2
```

After the fix:
```
dvl@dvl-mbp cpe2cve % echo "cpe:/a:foo:bar:*" | go run . -cpe 1 -e 1 -cve 1 -cache_size 1 ~/feeds/cve_20200424000000_nvd.json.gz
dvl@dvl-mbp cpe2cve %
```

cvefeed/cvecache.go

@@ -166,10 +166,10 @@ func (c *Cache) Get(cpes []*wfn.Attributes) []MatchResult {
 	cves.res = c.match(cpes)
 	cves.updateResSize(key)
 	c.mu.Lock()
-	c.size += cves.size
-	if c.MaxSize != 0 && c.size > c.MaxSize {
-		c.evict(int64(cacheEvictPercentage * float64(c.MaxSize)))
+	if c.MaxSize != 0 && c.size+cves.size > c.MaxSize {
+		c.evict(int64(cacheEvictPercentage*float64(c.MaxSize)) + cves.size)
 	}
+	c.size += cves.size
 	cves.evictionIndex = c.evictionQ.push(key)
 	c.mu.Unlock()
 	close(cves.ready)
@@ -231,7 +231,7 @@ func (c *Cache) matchDict(cpes []*wfn.Attributes, dict Dictionary) (results []Ma
 // evict the least recently used records untile nbytes of capacity is achieved or no more records left.
 // It is not concurrency-safe, c.mu should be locked before calling it.
 func (c *Cache) evict(nbytes int64) {
-	for c.size+nbytes > c.MaxSize {
+	for c.size > 0 && c.size+nbytes > c.MaxSize {
 		key := c.evictionQ.pop()
 		cd, ok := c.data[key]
 		if !ok { // should not happen