BibTeX catalogs: Strip unmatched braces

BibTeX catalogs: Strip unmatched braces to ensure to revent generation
of invalid .bib file. Fixes #1228905 [Calibre does not create a valid .bib file](https://bugs.launchpad.net/calibre/+bug/1228905)

Merge branch 'bibtex_fix' of https://github.com/alexbiddle/calibre

src/calibre/library/catalogs/bibtex.py

@@ -179,6 +179,8 @@ class BIBTEX(CatalogPlugin):
                     #\n removal
                     item = item.replace(u'\r\n',u' ')
                     item = item.replace(u'\n',u' ')
+                    # unmatched brace removal (users should use \leftbrace or \rightbrace for single braces)
+                    item = bibtexdict.stripUnmatchedSyntax(item, u'{', u'}')
                     #html to text
                     try:
                         item = html2text(item)

src/calibre/utils/bibtex.py

@@ -2902,3 +2902,29 @@ class BibTeX:
         """
         return self.utf8ToBibtex(u' and '.join([author for author in item]))
 
+    def stripUnmatchedSyntax(text, open_character, close_character):
+        """
+        Strips unmatched BibTeX syntax
+        """
+        stack = []
+        assert len(open_character) == 1 and len(close_character) == 1
+        remove = []
+        for i, ch in enumerate(text):
+            if ch == open_character:
+                stack.append(i)
+            elif ch == close_character:
+                try:
+                    stack.pop()
+                except IndexError:
+                    # Remove unmatched closing char
+                    remove.append(i)
+        # Remove unmatched opening chars
+        remove.extend(stack)
+
+        if remove:
+            text = list(text)
+            for i in sorted(remove, reverse=True):
+                text.pop(i)
+            text = ''.join(text)
+        return text
+