Fix T39777: Blender 2.65-2.7 Sapling Bug

Clamp branchL to min 0.0 (negative branch length does not make much sense, and
non-int power of negative numbers gives complex numbers...).

add_curve_sapling/__init__.py

@@ -25,7 +25,7 @@ bl_info = {
     "description": ("Adds a parametric tree. The method is presented by "
     "Jason Weber & Joseph Penn in their paper 'Creation and Rendering of "
     "Realistic Trees'."),
-    "warning": "length parameters may cause errors",  # used for warning icon and text in addons panel
+    #"warning": "length parameters may cause errors",  # used for warning icon and text in addons panel
     "wiki_url": "http://wiki.blender.org/index.php/Extensions:2.6/Py/"
         "Scripts/Curve/Sapling_Tree",
     "tracker_url": "https://developer.blender.org/T27226",

add_curve_sapling/utils.py

@@ -606,6 +606,7 @@ def addTree(props):
                         childStems = False
                     else:
                         childStems = leaves*shapeRatio(leafDist,p.offset/p.lengthPar)
+                branchL = max(branchL, 0.0)
                 # Determine the starting and ending radii of the stem using the tapering of the stem
                 startRad = min(p.radiusPar[0]*((branchL/p.lengthPar)**ratioPower), p.radiusPar[1])
                 endRad = startRad*(1 - taper[n])