Fix `-Wimplicit-fallthrough` warnings that seem to be new in GCC-7+

[sw17ch]

With GCC-7, the CFLAGS defined in the Makefile now detect warnings in pdjson.c. According to the GCC manual on the warning, a comment right before the following case statement that is /* FALLTHRU */ can be used to inform the compiler that the fall-through is intentional.

There are three cases in the is_legal_utf8 function that use fall-through of some sort. Two can be fixed with the comment and one can be fixed by making an implicit break an explicit one.

For reference, this is the output on my Ubuntu 16.04.6 LTS machine after installing gcc-7 and running make CC=gcc-7.

gcc-7 -c -std=c99 -pedantic -Wall -Wextra -Wno-missing-field-initializers -o pdjson.o pdjson.c
pdjson.c: In function ‘is_legal_utf8’:
pdjson.c:401:38: warning: this statement may fall through [-Wimplicit-fallthrough=]
         if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return 0;
             ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
pdjson.c:403:5: note: here
     case 3:
     ^~~~
pdjson.c:404:38: warning: this statement may fall through [-Wimplicit-fallthrough=]
         if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return 0;
             ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
pdjson.c:406:5: note: here
     case 2:
     ^~~~
pdjson.c:423:26: warning: this statement may fall through [-Wimplicit-fallthrough=]
             if (a < 0x80 || a > 0xBF) return 0;
                 ~~~~~~~~~^~~~~~~~~~~
pdjson.c:426:5: note: here
     case 1:
     ^~~~

[skeeto]

Looks good, thanks!