fix(radio): refuse "nan" for the airtime factor

"set af nan" is accepted and stores a NaN in airtime_factor, the duty cycle
budget the roles hand to their airtime accounting. strtof() consumes the
token, so the parse guard does not fire, and no ordered comparison is true for
a NaN, so "af < 0 || af > 9" cannot reject it. "set dutycycle" writes the same
field and has the same hole.

Both commands now check the text before it is parsed: after optional white
space and sign, a number has to start with a digit or a '.'. That refuses
"nan", "inf" and their variants, and keeps every spelling that was accepted
before (signs, leading white space, ".5"). The ranges and both reply strings
are unchanged.

The check is on the text rather than on the parsed float on purpose. nRF52
builds use -Ofast, and under -ffast-math the compiler may assume a float is
never NaN or infinite. A test on the value, even one that reads its bits, can
then be folded away: clang does exactly that with a bit test at -Ofast, while
arm-none-eabi-gcc keeps it. A character test does not depend on that.

src/helpers/CommonRadioPrefs.cpp

@@ -2,6 +2,22 @@
 #include "TxtDataHelpers.h"
 #include "Utils.h"
 #include "target.h"
+#include <ctype.h>
+
+namespace {
+
+// strtof() and atof() also read "nan" and "inf". A NaN cannot be refused by a
+// range check: no ordered comparison is true for it, and under -ffast-math,
+// which nRF52 builds use, the compiler may assume it never occurs and fold any
+// test on the parsed value away. So the text is checked before it is parsed:
+// after optional white space and sign, a number starts with a digit or a '.'.
+bool looksLikeANumber(const char* s) {
+  while (isspace((unsigned char)*s)) s++;
+  if (*s == '+' || *s == '-') s++;
+  return isdigit((unsigned char)*s) || *s == '.';
+}
+
+}  // namespace
 
 bool CommonRadioPrefs::getByKey(const char* key, char* value, size_t max_len) {
   if (strcmp(key, "fem_rxgain") == 0) {
@@ -69,7 +85,7 @@ bool CommonRadioPrefs::handleCommand(const char* command, uint32_t sender_timest
   if (memcmp(command, "set af ", 7) == 0) {
     char* end;
     float af = strtof(&command[7], &end);
-    if (end == &command[7] || af < 0 || af > 9) {
+    if (end == &command[7] || !looksLikeANumber(&command[7]) || af < 0 || af > 9) {
       strcpy(reply, "ERROR: af must be 0-9");
     } else {
       setAirtimeFactor(af);
@@ -87,7 +103,7 @@ bool CommonRadioPrefs::handleCommand(const char* command, uint32_t sender_timest
   }
   if (memcmp(command, "set dutycycle ", 14) == 0) {
     float dc = atof(&command[14]);
-    if (dc < 1 || dc > 100) {
+    if (!looksLikeANumber(&command[14]) || dc < 1 || dc > 100) {
       strcpy(reply, "ERROR: dutycycle must be 1-100");
     } else {
       setAirtimeFactor((100.0f / dc) - 1.0f);