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

formats/msgsm.h

@@ -1,4 +1,6 @@
 /* Conversion routines derived from code by guido@sienanet.it */
+#ifndef ASTERISK_MSGSM_H
+#define ASTERISK_MSGSM_H
 
 #define GSM_MAGIC 0xD
 
@@ -687,3 +689,5 @@ static inline void conv65( wav_byte * c, gsm_byte * d){
                         writeGSM_33(d+33);
 
 }
+
+#endif /* ASTERISK_MSGSM_H */