bugfix: read crash

Parsing some READ sentences with semantic errors crashed the
compiler. Fixed.

Author: boriel

tests/functional/read11.bas

@@ -0,0 +1,4 @@
+1 BORDER 0: PAPER 0: INK 9: BRIGHT 1: CLS
+10 LET spheres=2: IF spheres THEN DIM c(spheres,3): DIM r(spheres): DIM q(spheres)
+20 FOR k=1 TO spheres: READ c(k,1),c(k,2),c(k,3),r: LET r(k)=r: LET q(k)=r*r: NEXT k
+

zxbparser.py

@@ -1635,6 +1635,9 @@ def p_read(p):
     gl.DATA_IS_USED = True
     reads = []
 
+    if p[2] is None:
+        return
+
     for arg in p[2]:
         entry = arg.value
         if entry is None: