goto_rw: byte extracts may have a negative offset

A negative offset can be safely ignored as there is nothing to be read
from or to be written to.

Fixes: #2524

Author: tautschnig

regression/cbmc/full_slice3/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 --full-slice
 ^EXIT=0$

src/analyses/goto_rw.cpp

@@ -163,8 +163,9 @@ void rw_range_sett::get_objects_byte_extract(
       be.op().type(),
       be.id()==ID_byte_extract_little_endian,
       ns);
-    range_spect offset =
-      range_start + map.map_bit(numeric_cast_v<std::size_t>(*index));
+    range_spect offset = range_start;
+    if(*index > 0)
+      offset += map.map_bit(numeric_cast_v<std::size_t>(*index));
     get_objects_rec(mode, be.op(), offset, size);
   }
 }