Fixes #2518 -- weird behaviour of lookup_or_init (#2520)

* Allow lookup_or_init to return NULL (rather than just returning from the current function)
* Fixed a couple of bad edits found when running tests. Also fixed a bug in the
test runner. Also fixed a bug in libbcc where the python signature did not match
the actual implementation.
diff --git a/tools/lib/uobjnew.py b/tools/lib/uobjnew.py
index 85f5768..63dd80a 100755
--- a/tools/lib/uobjnew.py
+++ b/tools/lib/uobjnew.py
@@ -80,8 +80,10 @@
     struct val_t *valp, zero = {};
     key.size = size;
     valp = allocs.lookup_or_init(&key, &zero);
-    valp->total_size += size;
-    valp->num_allocs += 1;
+    if (valp) {
+        valp->total_size += size;
+        valp->num_allocs += 1;
+    }
     return 0;
 }
     """
@@ -98,8 +100,10 @@
     bpf_usdt_readarg(4, ctx, &size);
     bpf_probe_read(&key.name, sizeof(key.name), (void *)classptr);
     valp = allocs.lookup_or_init(&key, &zero);
-    valp->total_size += size;
-    valp->num_allocs += 1;
+    if (valp) {
+        valp->total_size += size;
+        valp->num_allocs += 1;
+    }
     return 0;
 }
     """
@@ -115,8 +119,10 @@
     u64 size = 0;
     bpf_usdt_readarg(1, ctx, &size);
     valp = allocs.lookup_or_init(&key, &zero);
-    valp->total_size += size;
-    valp->num_allocs += 1;
+    if (valp) {
+        valp->total_size += size;
+        valp->num_allocs += 1;
+    }
     return 0;
 }
     """
@@ -128,7 +134,9 @@
     bpf_usdt_readarg(1, ctx, &classptr);
     bpf_probe_read(&key.name, sizeof(key.name), (void *)classptr);
     valp = allocs.lookup_or_init(&key, &zero);
-    valp->num_allocs += 1;  // We don't know the size, unfortunately
+    if (valp) {
+        valp->num_allocs += 1;  // We don't know the size, unfortunately
+    }
     return 0;
 }
     """
@@ -146,7 +154,9 @@
     struct key_t key = { .name = "<ALL>" };
     struct val_t *valp, zero = {};
     valp = allocs.lookup_or_init(&key, &zero);
-    valp->num_allocs += 1;
+    if (valp) {
+        valp->num_allocs += 1;
+    }
     return 0;
 }
     """