[fidlfmt] Fix comment line sniffing

For comments with no content, i.e. "<opt-space>//" we were incorrectly
treating them as non-comment lines.

Test: added

FIDL-613 #done

Change-Id: I7e877658b567f612d01698acfde132bc4196aaab

zircon/system/host/fidl/include/fidl/formatter.h

@@ -261,7 +261,11 @@ private:
             // doc comments, which start with three slashes, should not
             // be treated as comments since they get internally converted
             // to attributes
-            return (i < static_cast<int>(str.size()) - 2) && str[i + 2] != '/';
+            if (str.size() == 2) {
+                return true;
+            } else {
+                return str[i + 2] != '/';
+            }
         } else {
             return false;
         }

zircon/system/utest/fidl-compiler/testdata/badformat.fidl.noformat

@@ -167,6 +167,11 @@ Before();
 ;
 
 
+// Some multiline
+//
+// comment with blank lines
+struct Hi{};
+
 
 // Comments at EOF
 

zircon/system/utest/fidl-compiler/testdata/goodformat.test.fidl

@@ -163,4 +163,10 @@ protocol POfMany
     -> After();
 };
 
+// Some multiline
+//
+// comment with blank lines
+struct Hi {
+};
+
 // Comments at EOF