Fix symbol lookup failure on the lowest-address symbol.

The `for` statement that looped backwards from `addrtab.upper_bound()`
was accidentally checking for `it == addrtab.begin()` _after_
decrementing `it`, not before. So after stepping the iterator
backwards to point at the first symbol in the map, that symbol wasn't
actually considered eligible to be the answer.

Author: statham-arm

lib/image.cpp

@@ -174,7 +174,8 @@ const Symbol *Image::find_symbol(Addr address) const
         return nullptr;
 
     // find the first symbol that contains or starts before 'address'
-    for (--it; it != addrtab.begin(); --it) {
+    while (it != addrtab.begin()) {
+        --it;
         const vector<const Symbol *> &syms = it->second;
 
         for (const Symbol *sym : syms) {