various files - fix some alerts raised by lgtm code analysis

This patch fixes several issues reported by the lgtm code analysis tool:

https://lgtm.com/projects/g/asterisk/asterisk

Not all reported issues were addressed in this patch. This patch mostly fixes
confirmed reported errors, potential problematic code points, and a few other
"low hanging" warnings or recommendations found in core supported modules.
These include, but are not limited to the following:

* innapropriate stack allocation in loops
* buffer overflows
* variable declaration "hiding" another variable declaration
* comparisons results that are always the same
* ambiguously signed bit-field members
* missing header guards

Change-Id: Id4a881686605d26c94ab5409bc70fcc21efacc25

codecs/ex_ulaw.h

@@ -7,6 +7,9 @@
  *
  */
 
+#ifndef ASTERISK_EX_ULAW_H
+#define ASTERISK_EX_ULAW_H
+
 static uint8_t ex_ulaw[] = {
 	0x00, 0x03, 0x06, 0x09, 0x0c, 0x0f, 0x12, 0x15,
 	0x10, 0x18, 0x1b, 0x1e, 0x21, 0x24, 0x27, 0x2a,
@@ -36,3 +39,5 @@ static struct ast_frame *ulaw_sample(void)
 
 	return &f;
 }
+
+#endif /* ASTERISK_EX_ULAW_H */