Fix out-of-bounds error when no threads were needed for a zoom level

.travis.yml

@@ -43,7 +43,7 @@ matrix:
     # debug+leak+address-sanitizer build
     - os: linux
       compiler: clang
-      env: CLANG_VERSION='3.8.0' BUILDTYPE=Debug ASAN_OPTIONS=detect_leaks=1 CC="clang-3.8" CXX="clang++-3.8" CXXFLAGS="-fsanitize=address" CFLAGS="-fsanitize=address" LDFLAGS="-fsanitize=address" FEWER=true
+      env: CLANG_VERSION='3.8.0' BUILDTYPE=Debug ASAN_OPTIONS=detect_leaks=1 CC="clang-3.8" CXX="clang++-3.8" CXXFLAGS="-fsanitize=address,undefined" CFLAGS="-fsanitize=address,undefined" LDFLAGS="-fsanitize=address,undefined" FEWER=true
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test' ]

tests/onefeature/-zg_--drop-densest-as-needed.json

@@ -0,0 +1,13 @@
+{ "type": "FeatureCollection", "properties": {
+"bounds": "-180.000000,0.800000,180.000000,1.000000",
+"center": "-179.989014,1.000000,14",
+"description": "tests/onefeature/-zg_--drop-densest-as-needed.json.check.mbtiles",
+"format": "pbf",
+"json": "{\"vector_layers\": [ { \"id\": \"in\", \"description\": \"\", \"minzoom\": 0, \"maxzoom\": 14, \"fields\": {} } ],\"tilestats\": {\"layerCount\": 1,\"layers\": [{\"layer\": \"in\",\"count\": 1,\"geometry\": \"LineString\",\"attributeCount\": 0,\"attributes\": []}]}}",
+"maxzoom": "14",
+"minzoom": "0",
+"name": "tests/onefeature/-zg_--drop-densest-as-needed.json.check.mbtiles",
+"type": "overlay",
+"version": "2"
+}, "features": [
+] }

tests/onefeature/in.json

@@ -0,0 +1 @@
+{"":"","":[{"":"","":{"":[]},"":{"":""}},{"":"","type":"LineString","coordinates":[[1000,0.8],[900,1]]}

tile.cpp

@@ -2363,6 +2363,9 @@ int traverse_zooms(int *geomfd, off_t *geom_size, char *metabase, char *stringpo
 		if (threads >= (1U << 30)) {
 			threads = 1U << 30;
 		}
+		if (threads < 1) {
+			threads = 1;
+		}
 
 		// Assign temporary files to threads