Ensure the flow through switch statements is clear

Ensure case clauses:
*   Terminate with an unconditional break, return or goto statement.
*   Use conditional break, return or goto statements as long as the end
    of the case clause is unreachable; such case clauses must terminate
    with assert(0) /* Unreachable */ or an unconditional  __dead2 function
    call
*   Only fallthough when doing otherwise would result in less
    readable/maintainable code; such case clauses must terminate with a
    /* Fallthrough */ comment to make it clear this is the case and
    indicate that a fallthrough is intended.

This reduces the chance of bugs appearing due to unintended flow through a
switch statement

Change-Id: I70fc2d1f4fd679042397dec12fd1982976646168
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>

lib/libc/printf.c

@@ -166,6 +166,7 @@ loop:
 					padn = (padn * 10) + (ch - '0');
 					fmt++;
 				}
+				assert(0); /* Unreachable */
 			default:
 				/* Exit on any other format specifier */
 				return -1;

lib/libc/snprintf.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#include <assert.h>
 #include <debug.h>
 #include <platform.h>
 #include <stdarg.h>
@@ -117,6 +118,7 @@ int snprintf(char *s, size_t n, const char *fmt, ...)
 				ERROR("snprintf: specifier with ASCII code '%d' not supported.",
 				      *fmt);
 				plat_panic_handler();
+				assert(0); /* Unreachable */
 			}
 			fmt++;
 			continue;