Possible fix for bug #18648: one armature disappears when joining

armatures. Not sure it actually fixes the problem because I can't
test this on Win 64, but this fixes use of unitialized memory at
least. Mat4MulMat34 only sets the 3x3 part of the 4x4 matrix, which
may lead to a NaN bone roll value.

source/blender/src/editarmature.c

@@ -715,6 +715,7 @@ int join_armature(void)
 						VecSubf(delta, curbone->tail, curbone->head);
 						vec_roll_to_mat3(delta, curbone->roll, temp);
 						
+						Mat4One(premat); /* Mat4MulMat34 only sets 3x3 part */
 						Mat4MulMat34(premat, temp, mat);
 						
 						Mat4MulVecfl(mat, curbone->head);