Increase string2ld's buffer size (and fix HINCRBYFLOAT)

The string representation of `long double` may take
up to ~5000 chars (see PR #3745).

Before this fix HINCRBYFLOAT would never overflow (since
the string could not exceed 256 chars). Now it can.

src/t_hash.c

@@ -615,6 +615,10 @@ void hincrbyfloatCommand(client *c) {
     }
 
     value += incr;
+    if (isnan(value) || isinf(value)) {
+        addReplyError(c,"increment would produce NaN or Infinity");
+        return;
+    }
 
     char buf[MAX_LONG_DOUBLE_CHARS];
     int len = ld2string(buf,sizeof(buf),value,1);