Add "clang-tidy" integration and fix outstanding issues

clang-tidy is a C linter built as part of the LLVM project. You can run
it against ashuffle sources by running:

```
meson -Dtidy_mode=true build
ninja -C build clang-tidy
```

.clang-tidy

@@ -0,0 +1,30 @@
+---
+Checks:          'clang-diagnostic-*,clang-analyzer-*,-clang-analyzer-valist.Uninitialized,-clang-analyzer-security.insecureAPI.*'
+WarningsAsErrors: ''
+HeaderFilterRegex: ''
+AnalyzeTemporaryDtors: false
+FormatStyle:     none
+User:            josh
+CheckOptions:    
+  - key:             google-readability-braces-around-statements.ShortStatementLines
+    value:           '1'
+  - key:             google-readability-function-size.StatementThreshold
+    value:           '800'
+  - key:             google-readability-namespace-comments.ShortNamespaceLines
+    value:           '10'
+  - key:             google-readability-namespace-comments.SpacesBeforeComments
+    value:           '2'
+  - key:             modernize-loop-convert.MaxCopySize
+    value:           '16'
+  - key:             modernize-loop-convert.MinConfidence
+    value:           reasonable
+  - key:             modernize-loop-convert.NamingStyle
+    value:           CamelCase
+  - key:             modernize-pass-by-value.IncludeStyle
+    value:           llvm
+  - key:             modernize-replace-auto-ptr.IncludeStyle
+    value:           llvm
+  - key:             modernize-use-nullptr.NullMacros
+    value:           'NULL'
+...
+

meson.build

@@ -21,15 +21,28 @@ low_dep_sources = [
 
 sources = low_dep_sources + high_dep_sources
 
+executable_sources = sources + ['src/main.c']
+
 clientlib = dependency('libmpdclient')
 
 ashuffle = executable(
 	'ashuffle',
-	sources + ['src/main.c'],
+	executable_sources,
 	dependencies: clientlib,
 	install: true
 )
 
+if get_option('tidy_mode')
+    clang_tidy = run_target('clang-tidy',
+        command : ['clang-tidy',
+                   '-p', meson.build_root(),
+                  ] + executable_sources,
+    )
+endif
+
+# We only generate tests if we are *not running in tidy mode
+if not get_option('tidy_mode')
+
 subdir('t')
 
 test_c_args = [
@@ -62,3 +75,5 @@ foreach test_name, test_sources : tests
     )
     test(test_name, test_exe)
 endforeach
+
+endif # tidy_mode

meson_options.txt

@@ -0,0 +1 @@
+option('tidy_mode', type : 'boolean', value : 'false')

src/ashuffle.c

@@ -78,7 +78,7 @@ bool ruleset_accepts_uri(struct mpd_connection * mpd,
         /* even though we're searching for a single song, libmpdclient
          * still acts like we're reading a song list. We read an aditional
          * element to convince MPD this is the end of the song list. */
-        song = mpd_recv_song(mpd);
+        (void) mpd_recv_song(mpd);
     } else {
         fprintf(stderr, "Song uri '%s' not found.\n", uri);
     }
@@ -102,11 +102,10 @@ int build_songs_file(struct mpd_connection * mpd, struct list * ruleset,
         }
 
         /* if this line has terminating newline attached, set it
-         * to null and decrement the length (effectively removing
-         * the newline). */
+         * to null (effectively removing the newline). */
         if (uri[length - 1] == '\n') {
-            uri[length - 1] = '\0';
             length -= 1;
+            uri[length] = '\0';
         }
 
         if ((check && ruleset_accepts_uri(mpd, ruleset, uri)) || (! check)) {

src/util.c

@@ -34,6 +34,8 @@ char * xsprintf(const char * fmt, ...) {
         abort();
     }
 
+    va_end(rest);
+
     return res;
 }
 
@@ -47,5 +49,7 @@ void die(const char * fmt, ...) {
     strcat(fmt_nl, "\n");
 
     vfprintf(stderr, fmt_nl, rest);
+
+    va_end(rest);
     exit(1);
 }