Fix OOB access due to multiple backspaces

Commit 419ca82d57 (Fix m17n backspace handling causes out-of-bounds
write in checkType) introduced an incomplete fix.

In function checkType we store the length of the previous multi-char
character in a buffer plens_buffer with pointer plens pointing to the
current position inside the buffer. When encountering a backspace plens
is set to the previous position without a bounds check. This will lead
to plens being out of bounds if we get more backspaces than we have
processed multi-char characters before.

If we are at the beginning of the buffer do not decrement and set plen
(the current length) to 0.

This also fixes GH Issue #270 [BUG] Out of bound read in Strnew_size ,
Str.c:61

If the above explanation does sound weird it's because I didn't fully
grok that function. :-)

etc.c

@@ -393,7 +393,10 @@ checkType(Str s, Lineprop **oprop, Linecolor **ocolor)
 			    if (color)
 				color -= plen;
 #endif
-			    plen = *(--plens);
+			    if (plens == plens_buffer)
+				plen = 0;
+			    else
+				plen = *(--plens);
 			    str += 2;
 			}
 		    }
@@ -419,7 +422,10 @@ checkType(Str s, Lineprop **oprop, Linecolor **ocolor)
 			    if (color)
 				color -= plen;
 #endif
-			    plen = *(--plens);
+			    if (plens == plens_buffer)
+				plen = 0;
+			    else
+				plen = *(--plens);
 			    str++;
 			}
 #else