fix: Fix -Wanalyzer-deref-before-check warning in cmd_ac_complete

Adding an explicit NULL check for 'input' at the start of
cmd_ac_complete to prevent a potential crash and resolve
a GCC static analyzer warning.

Quirk Explanation:
The analyzer found  a "deref-before-check" warning for:
`if ((strncmp(input, "/", 1) == 0) && (!strchr(input, ' ')))`

The analyzer interpreted 'strchr(input, ' ')' as a point where the
validity of 'input' is being questioned (a "check"), even though it is
actually checking the return value. This is due to GCC's internal model
of certain standard string functions having a 'nonnull' attribute or
being categorized as pointer "interrogations" in its state machine.

Because 'strncmp' dereferences 'input' earlier in the same line, the
analyzer saw a logical contradiction: "You treat it as safe in strncmp,
but then you call a function (strchr) that 'requires/checks' it to be
safe, implying you weren't sure."

src/command/cmd_ac.c

@@ -1545,6 +1545,10 @@ cmd_ac_remove_form_fields(DataForm* form)
 char*
 cmd_ac_complete(ProfWin* window, const char* const input, gboolean previous)
 {
+    if (!input) {
+        return NULL;
+    }
+
     char* found = NULL;
     // autocomplete command
     if ((strncmp(input, "/", 1) == 0) && (!strchr(input, ' '))) {