Fix segfaults with malformed input and wrong command line arguments

Fixes:
NULL dereference.

Steps to reproduce:
echo L x | tests/ampctl
echo L x | tests/rigctl
echo L x | tests/rotctl

tests/ampctl_parse.c

@@ -1161,7 +1161,7 @@ int ampctl_parse(AMP *my_amp, FILE *fin, FILE *fout, char *argv[], int argc)
 
                 rp_getline(pmptstr);
 
-                if (!(strcmp(input_line, "")))
+                if (!input_line || !(strcmp(input_line, "")))
                 {
                     fprintf(fout, "? for help, q to quit.\n");
                     fflush(fout);

tests/rigctl_parse.c

@@ -1544,7 +1544,7 @@ readline_repeat:
 
                 rp_getline(pmptstr);
 
-                if (!(strcmp(input_line, "")))
+                if (!input_line || !(strcmp(input_line, "")))
                 {
                     fprintf(fout, "? for help, q to quit.\n");
                     fflush(fout);

tests/rotctl_parse.c

@@ -1221,7 +1221,7 @@ int rotctl_parse(ROT *my_rot, FILE *fin, FILE *fout, char *argv[], int argc,
 
                 rp_getline(pmptstr);
 
-                if (!(strcmp(input_line, "")))
+                if (!input_line || !(strcmp(input_line, "")))
                 {
                     fprintf(fout, "? for help, q to quit.\n");
                     fflush(fout);