Fix off-by-one error in MIME pattern matching

This adds a size to the test webp file, since the error fixed occurs
when the test object is the same length as the matched pattern, and
is not equal to the pattern.

components/net/mime_classifier.rs

@@ -294,7 +294,7 @@ impl ByteMatcher {
         } else if data == self.pattern {
             Some(self.pattern.len())
         } else {
-            data[..data.len() - self.pattern.len()].iter()
+            data[..data.len() - self.pattern.len() + 1].iter()
                 .position(|x| !self.leading_ignore.contains(x))
                 .and_then(|start|
                     if data[start..].iter()
@@ -316,15 +316,19 @@ impl MIMEChecker for ByteMatcher {
     }
 
     fn validate(&self) -> Result<(), String> {
+        if self.pattern.len() == 0 {
+            return Err(format!(
+                "Zero length pattern for {}/{}",
+                self.content_type.0, self.content_type.1
+                ))
+        }
         if self.pattern.len() != self.mask.len() {
-            Err(format!(
-                "Unequal pattern and mask length for {}/{}", 
+            return Err(format!(
+                "Unequal pattern and mask length for {}/{}",
                 self.content_type.0, self.content_type.1
             ))
         }
-        else {
-            Ok(())
-        }
+        Ok(())
     }
 }