Fix rangeShift calculations for count=0 in sfnt, cmap, and kern
diff --git a/Lib/fontTools/ttLib/sfnt.py b/Lib/fontTools/ttLib/sfnt.py
index c6bc93a..95679e9 100644
--- a/Lib/fontTools/ttLib/sfnt.py
+++ b/Lib/fontTools/ttLib/sfnt.py
@@ -483,7 +483,7 @@
exponent = maxPowerOfTwo(n)
searchRange = (2 ** exponent) * 16
entrySelector = exponent
- rangeShift = n * 16 - searchRange
+ rangeShift = max(0, n * 16 - searchRange)
return searchRange, entrySelector, rangeShift
diff --git a/Lib/fontTools/ttLib/tables/_c_m_a_p.py b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
index d0c6c41..a3fb6cc 100644
--- a/Lib/fontTools/ttLib/tables/_c_m_a_p.py
+++ b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
@@ -781,7 +781,7 @@
maxExponent = maxPowerOfTwo(segCount)
searchRange = 2 * (2 ** maxExponent)
entrySelector = maxExponent
- rangeShift = 2 * segCount - searchRange
+ rangeShift = max(0, 2 * segCount - searchRange)
charCodeArray = array.array("H", endCode + [0] + startCode)
idDeltaArray = array.array("H", idDelta)
diff --git a/Lib/fontTools/ttLib/tables/_k_e_r_n.py b/Lib/fontTools/ttLib/tables/_k_e_r_n.py
index 9fa9dd8..b8e60d3 100644
--- a/Lib/fontTools/ttLib/tables/_k_e_r_n.py
+++ b/Lib/fontTools/ttLib/tables/_k_e_r_n.py
@@ -118,7 +118,7 @@
nPairs = len(self.kernTable)
entrySelector = sfnt.maxPowerOfTwo(nPairs)
searchRange = (2 ** entrySelector) * 6
- rangeShift = (nPairs - (2 ** entrySelector)) * 6
+ rangeShift = max(0, (nPairs - (2 ** entrySelector)) * 6)
data = struct.pack(">HHHH", nPairs, searchRange, entrySelector, rangeShift)
# yeehee! (I mean, turn names into indices)