Fix the latent tail from the Common/Core review

Buffer::PeekAll resized the destination and then appended, so it returned twice the
data with zeros in front. No callers today.

The assert dialog's caption was a function-local static, so it froze at whichever
kind of assert fired first in the session - a later Critical Assert would still be
titled 'Debug Assert'. Made it a plain const char *.

containsNoCase passed a possibly-negative char to std::toupper, which is UB for
anything but EOF; non-ASCII bytes get here from ELF symbol names.

ApplyDeadzoneXY's circular path lacked the deadzone >= 0.99f guard its own sibling
ApplyDeadzoneAxis has one function above, so it divided by ~0.

Common/StringUtils.cpp

@@ -86,7 +86,11 @@ long parseLong(std::string s) {
 }
 
 bool containsNoCase(std::string_view haystack, std::string_view needle) {
-	auto pred = [](char ch1, char ch2) { return std::toupper(ch1) == std::toupper(ch2); };
+	// toupper is UB for negative values other than EOF, and char is signed here - non-ASCII bytes
+	// reach this from e.g. ELF symbol names.
+	auto pred = [](char ch1, char ch2) {
+		return std::toupper((unsigned char)ch1) == std::toupper((unsigned char)ch2);
+	};
 	auto found = std::search(haystack.begin(), haystack.end(), needle.begin(), needle.end(), pred);
 	return found != haystack.end();
 }