Fix mathinfo (#137)

* Proposed fix.

* It seems addition is different from subtractions.

Author: LettError

Lib/fontMath/mathInfo.py

@@ -60,9 +60,17 @@ def _processMathOne(self, copiedInfo, otherInfo, ptFunc, func):
                 else:
                     v = self._processMathOneNumber(a, b, func)
             elif a is not None and b is None:
-                v = a
+                #v = a
+                if func == add:
+                    v = a
+                else:
+                    v = 0
             elif b is not None and a is None:
-                v = b
+                #v = b
+                if func is add:
+                    v = b
+                else:
+                    v = 0
             if v is not None:
                 setattr(copiedInfo, attr, v)
         # special attributes