symproc: invalidate LHS objects on invalid dereference

... to simulate that invalid operations may by chance write to the
target object within the allocated bounds.  Nevertheless, the same
situation may happen with `executeMemset()`, `executeMemmove()`, etc.

With this change and `SE_ERROR_RECOVERY_MODE == 2`, Predator reports
one more error in the following example:
```
% nl -ba xxx.c
     1  #include <verifier-builtins.h>
     2
     3  int main()
     4  {
     5      int a[10] = {0};
     6      int i = __VERIFIER_nondet_int();
     7      a[i] = 11;
     8      a[a[1]] = 1;
     9  }

% ./slgcc xxx.c
Trying to compile xxx.c ... OK
Running Predator ...
xxx.c:7:10: error: invalid dereference
xxx.c:8:13: error: invalid dereference
cl/cl_easy.cc:83: note: clEasyRun() took 0.000 s
FAILED
```

Related: #102

sl/symproc.cc

@@ -1485,6 +1485,21 @@ bool SymProc::lhsFromOperand(FldHandle *pLhs, const struct cl_operand &op)
         return true;
     }
 
+    if (2 <= GlConf::data.errorRecoveryMode) {
+        // if the dereference failed due to out of range (or unknown) offset,
+        // try to resolve the allocated object and invalidate its contents in
+        // order to detect more errors in one run with full error recovery
+        const TObjId obj = this->objByVar(op);
+        if (sh_.isValid(obj)) {
+            const UniformBlock ub = {
+                /* off      */  0,
+                /* size     */  sh_.objSize(obj).hi,
+                /* tplValue */  sh_.valCreate(VT_UNKNOWN, VO_UNKNOWN)
+            };
+            sh_.writeUniformBlock(obj, ub);
+        }
+    }
+
     return false;
 }