WebPRescalerGetScaledDimensions: round scaled dimension up

This is to prevent resizing to dimension 0

+ added some safety checks about src_width > 0 and src_height > 0

BUG=webp:418

Change-Id: Ic04a53ad26455d80538bc8681882a554fca2a340

src/utils/rescaler_utils.c

@@ -84,14 +84,14 @@ int WebPRescalerGetScaledDimensions(int src_width, int src_height,
     int height = *scaled_height;
 
     // if width is unspecified, scale original proportionally to height ratio.
-    if (width == 0) {
+    if (width == 0 && src_height > 0) {
       width =
-          (int)(((uint64_t)src_width * height + src_height / 2) / src_height);
+          (int)(((uint64_t)src_width * height + src_height - 1) / src_height);
     }
     // if height is unspecified, scale original proportionally to width ratio.
-    if (height == 0) {
+    if (height == 0 && src_width > 0) {
       height =
-          (int)(((uint64_t)src_height * width + src_width / 2) / src_width);
+          (int)(((uint64_t)src_height * width + src_width - 1) / src_width);
     }
     // Check if the overall dimensions still make sense.
     if (width <= 0 || height <= 0) {