Incorrect label in map of dot files in xhtml (#7840)

When a filename of a file starts with a digit the mapping of the resulting dot files results in message like:
```
Syntax of value for attribute id of map is not valid
```
an id cannot start with a digit, so an "a" is placed in front of it (unconditionally to overcome problems with a double label id i.e filename 087.cpp and a087.cpp).

1 files changed, 10 insertions, 0 deletions

diff --git a/src/util.cpp b/src/util.cpp
index 1624b1c..a298ace 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -5278,6 +5278,16 @@ QCString convertToId(const char *s)
   return growBuf.get();
 }
 
+/*! Some strings have been corrected but the requirement regarding the fact
+ *  that an id cannot have a digit at the first position. To overcome problems
+ *  with double labels we always place an "a" in front
+ */
+QCString correctId(QCString s)
+{
+  if (s.isEmpty()) return s;
+  return "a" + s;
+}
+
 /*! Converts a string to an XML-encoded string */
 QCString convertToXML(const char *s, bool keepEntities)
 {